Help - Carb Backfire

[72moneypit]

I was driving to work today and started hearing/feeling carb backfires. I got nervous and exited the highway, and it seemed to drive OK. When I re-entered the highway, it started again... at about 55-60 MPH.

My question: What would cause a carb backfire at 55+ MPH, but otherwise it runs fine? I have to wonder if the outside temp (about 105) is contributing - I've not driven the truck in such hot weather since rebuilding the engine and installing the new carb.

Any ideas? Thanks!