Whats causing this? Joystick Problem

My Saitek X45 works fine for a while then eventually, within 10 minutes or so, I can only achieve about 80% power from the throttle.A quick check of the calibration confirms this. The colored bar only moves about 3/4 up.BUT- If I unplug the Saitek "usb" from my computer,replug it back in, its works fine but after a short while will again only go 3/4 power.Any ideas why disconnecting it makes the throttle operate properly but only for a short while? All other functions on both stick and throttle work correctly. Just the throttle.