I have had a post running in LEDs and Multiplexing which describes a project involving 20 light boxes each with 12 channels of LED lighting that I control with PWM. I had a nice code running on the Uno board but because I needed extra PWM channels I upgraded to the Mega 2560. And here is my problem; the only way I can upload any code to the Mega is by shutting down my computer and restarting, then I have only one chance to communicate with the board. Any additional attempts to upload produce a long string of time out messages.
The fact that I have success with Uno board while the Mega board struggles leads me to think it is a bug in the software onboard the Mega. The computer, USB lead, Arduino IDE, code and test LED hardware are all one and the same. Both boards appear to be official Arduino product.
One point of difference between the boards is that the LED marked L under pin 13 remains on with the Mega while the sketch is running, but it does not stay on while running on the Uno. During an attempt to upload a new sketch this LED will flicker briefly until the sketch size message is displayed and then it stays on. The old sketch continues running. There will be occasional flashes on the TX and RX LEDs until the attempted upload is finally abandoned with the message
avrdude: stk500v2_getsync(): timeout communicating with programmer
avrdude: stk500v2_command(): failed miserably to execute command 0x11
avrdude: stk500v2_disable(): failed to leave programming mode
Any advice will be greatly appreciated. I have a limited intelligence.