Hi,
When I've had issues with PK3's they seem to all have been related to USB power supply issues.
If used on a laptop make sure you have the computer powered by the mains adapter and try another, short, high quality USB cable. Last time I had issues I had two laptops side by side, the PK3 worked fine on one of them but not on the other. Swapped the USB cable to a better one and everything's fine... (No, I was not powering my circuit "thru" the PK3, just the PK3 itself).

If you're using a desktop computer (which it sounds like you are) you could possibly try a powered USB hub.

Not saying this IS the problem but I've run into a couple of times.

/Henrik.