After uploading the first sketch or two, the Arduino will no longer accept uploads and gives the error message: "avrdude: stk500_getsync(): not in sync: resp=0x00". The only way I've figured out how to get it working again is to reboot the laptop. This, of course, is very annoying and makes the system virtually unusable. I've tried a number of fixes that were suggested on the various posts including the loop-back test which tested good despite being unable to upload sketches.
Any idea what's going on here? This is the Arduino UNO ATmega328P.