old sketches not being overwriting on board


I seem to be having a problem with parts of old sketches that have been uploaded persisting in new uploads. eg: an earlier sketch had a custom character, now i've uploaded a completely new sketch and the custom character has popped up. the problem has now got to the stage that the sketches a just coming out garbled on the LCD.

Is there some way to ensure the memory on the board is wiped clean? Or is there some other problem i'm missing?

Details; 1280mega 4x20 LCD (web4robot) 16 key keypad

Cheers, Paul.

never mind, it seems to be a problem with arduino 21, i go back to using 18 and it runs fine. don't know why, but i'll stay using 18.

Cheers anyway.