when you say your brakelights are lit is it all 3(third brakelight too)?
I had an issue with mine, that the truck would arbitrarily light the brakelights(not the third brakelight) with the truck parked and occasionally while driving. in my case my brake controller was sending power thru the sensing circuit intermittantly telling the computer that I was stepping on the brakes. the third brakelight is on a different circuit in the brakeswitch so it would never light with the others. If you have a brake controller try disconnecting it and see if your problem goes away.