1997 Dodge Caravan ENGINE BACKFIRE

My Grand caravan w/3.3 176000 miles started this about 2 weeks ago.M I drive 50 miles each way to work and back. When I leave in the AM all good, but when I have been driving awhile and the I slow down for a light or whatever, when I accelerate, there is a hesitation and some backfire from the engine area. Once I get going its fine, but each time I slow down, the same thing happens. Then, after the engine is cool and I am going home, its all good until I have to slow down. Im on the interstate at about 75mph for at least 45 minutes or so. I have replaced spark plugs and air filter. HELP!