Error uploading sketch en Mac OS

Hola a todos, os agradecería que me ayudarais para averiguar porque el IDE no va en mi Mac.
El primer día funcionó perfectamente y al día siguiente al retomar la tarea ya me daba el famoso y odioso error de "avrdude: stk500_recv(): programmer is not responding". Por mucho que he leido en los foros no consigo encontrar la solución y el caso es que otro dia funcionó también pero tras varias cargas de software volvió a ocurrir lo mismo y hasta hoy.
He probado todo lo que recomiendan, el test del puerto serie es correcto. Arduino funciona prefectamente en cualquier otro ordenador con Windows. Pero en mi Mac no lo consigo.
Los puertos serie que me ofrece son los que a todo el mundo /dev/cu.modemusb....y /dev/tty.modemusb...
y en cualquiera de los dos pasa el test correctamente.
Cuando mando un upload, el AVRDUDE mantiene el dialogo inicial con Arduino e incluso dice que está todo ok pero cuando manda el archivo .hex en el primer bloque de datos Arduino parece que se queda congelado y deja de contestar por lo que el AVRDUDE da el maldito mensaje de error.
Este es el volcado del AVRDUDE:

avrdude: Version 5.11, compiled on Sep  2 2011 at 18:52:52
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2009 Joerg Wunsch

         System wide configuration file is "/Applications/Arduino.app/Contents/Resources/Java/hardware/tools/avr/etc/avrdude.conf"
         User configuration file is "/Users/myself/.avrduderc"
         [color=blue]User configuration file does not exist or is not a regular file, skipping[/color][color=red]<---Podría ser este el problema??????[/color]

         Using Port                    : /dev/cu.usbmodemfd131
         Using Programmer              : arduino
         Overriding Baud Rate          : 115200
avrdude: Send: 0 [30]   [20] 
avrdude: Send: 0 [30]   [20] 
avrdude: Send: 0 [30]   [20] 
avrdude: Recv: . [14] 
avrdude: Recv: . [10] 
         AVR Part                      : ATMEGA328P
         Chip Erase delay              : 9000 us
         PAGEL                         : PD7
         BS2                           : PC2
         RESET disposition             : dedicated
         RETRY pulse                   : SCK
         serial program mode           : yes
         parallel program mode         : yes
         Timeout                       : 200
         StabDelay                     : 100
         CmdexeDelay                   : 25
         SyncLoops                     : 32
         ByteDelay                     : 0
         PollIndex                     : 3
         PollValue                     : 0x53
         Memory Detail                 :

blah-blah....

avrdude: AVR device initialized and ready to accept instructions

Reading | avrdude: Send: u [75]   [20] 
avrdude: Recv: . [14] . [1e] . [95] . [0f] . [10] 
################################################## | 100% 0.00s

blah....blah...

avrdude: reading input file "/var/folders/4m/8qq8bzw56sl7r0m1g492qc3h0000gn/T/build5901878382664752600.tmp/AnalogReadSerial.cpp.hex"
avrdude: writing flash (2596 bytes):

Writing | avrdude: Send: U [55] . [00] . [00]   [20] 
avrdude: Recv: . [14] 
avrdude: Recv: . [10] 
avrdude: Send: d [64] . [00] . [80] F [46] . [0c] . [94] 5 [35] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] y [79] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] x [78] . [01] . [0c] . [94] . [c6] . [01] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] . [0c] . [94] ] [5d] . [00] 2 [32] . [03] . [11] $ [24] . [1f] . [be] . [cf] . [ef] . [d8] . [e0] . [de] . [bf] . [cd] . [bf] . [11] . [e0] . [a0] . [e0] . [b1] . [e0] . [e2] . [e1] . [fa] . [e0]   [20] 
avrdude: Recv: . [14] 
avrdude: Recv: . [10] 
##avrdude: Send: U [55] @ [40] . [00]   [20] 
avrdude: Recv: . [14] 
avrdude: Recv: . [10] 
avrdude: Send: d [64] . [00] . [80] F [46] . [02] . [c0] . [05] . [90] . [0d] . [92] . [a2] 1 [31] . [b1] . [07] . [d9] . [f7] . [11] . [e0] . [a2] . [e1] . [b1] . [e0] . [01] . [c0] . [1d] . [92] . [a5] < [3c] . [b1] . [07] . [e1] . [f7] . [10] . [e0] . [ca] . [e6] . [d0] . [e0] . [04] . [c0] " [22] . [97] . [fe] . [01] . [0e] . [94] . [03] . [05] . [c8] 6 [36] . [d1] . [07] . [c9] . [f7] . [0e] . [94] . [88] . [03] . [0c] . [94] . [07] . [05] . [0c] . [94] . [00] . [00] . [8e] . [e0] . [0e] . [94] U [55] . [01] . [bc] . [01] . [83] . [ea] . [91] . [e0] J [4a] . [e0] P [50] . [e0] . [0e] . [94] . [a1] . [04] a [61] . [e0] p [70] . [e0] . [80] . [e0] . [90] . [e0] . [0e] . [94] . [c1] . [00] . [08] . [95] . [83] . [ea] . [91] . [e0] @ [40] . [e8] U [55] . [e2] ` [60] . [e0] p [70] . [e0] . [0e] . [94] . [01] . [02] . [08] . [95] . [1f] . [92] . [0f] . [92] . [0f] . [b6] . [0f] . [92] . [11] $ [24] / [2f] . [93] ? [3f] . [93]   [20] 
avrdude: ser_recv(): programmer is not responding
avrdude: stk500_recv(): programmer is not responding

Gracias por vuestra ayuda....