Backfiring Problem

I have a 1995 4.3L 6 262 z- code TBI motor. What is happening is that when i am at about 75 or 80 and hold it at that speed for10 to 15 minutes and lett off the gas to let the rpms slow the truck down, the motor backfires with a loud bang like a shotgun and a small flame shoots out the pipes. Timing is set correctly and just did tuneup on the motor. Also just cleaned the Fuel Injectors manually. Any suggestions?

Any help greatly appreciated.