[Bill Bess]

HMMMMMM! check the tail light sockets..maybe one of them is grounding out the stop light wire feed when it's hot. Loose connection..touching the side of the bulb socket?

Get another stop light set up to use as a test device, that will eliminate the tail lights as a potential problem. Also check the stop light switch for it's connection and proper funtion...process of elimination.

Good luck, Bill