Mega 2560 Upload problem

Hi, I have problem with Arduino sketch upload. I can upload skech only once after I plug Arudino into computer via USB, after that if I try upload sketch again I get this:

avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_getsync(): timeout communicating with programmer

Even If I eject USB and plug it again, I can't upload my skech. I have to keep arduino turned longer time and than it starts to work. But program, which Is already uploaded in Arduino works even if uploading is imposible. Do you think, that if I'll try (and is USBasp USBISP 3.3V / 5V AVR Programmer enought to do that) burn bootloader again it will help? Sorry about my English and thanks for help.

mmm... You don't happen to be using 1.6.8 on Windows do you?

That has a really bad bug that causes boards to continually reset while plugged in if the IDE is running on windows, which in turn can result in communication problems.

1.6.5r5 was the last "good" release.

I was using 1.6.8 and there was no problem. It started few days after I had connected Ethernet W5100 R3, but it can be just coincidence... I read about that issues with IDE, so I tried 1.6.6 too, but it's same. I noticed, that I maybe few times shorted Ethernet socket with USB female, I'l but programmer