I agree with Tonik. Unlikely that it's actually overheating. Given the rapidity that your seeing temp swings, I seriously doubt it's your t-stat, either. Most likely the sensor, but could be just as easily be a loose/corroded connection. Find the sensor and trace the wiring back thru the loom, pull each junction apart and put some dielectric grease on them. If that doesn't remedy the problem, the sensor should be an easy and cheap replace. Not sure what the high idle was all about, but electric gremlins can cause a plethora of problems in modern FI bikes, and unfortunately, Trumpets have always been notorious for them.