convertor slippage i bet. is that a 292 comp magnum cam in that? if so, any less convertor than you have now will probably make the car sluggish. my own car with 440 and 292 magnum picked up 3 tenths with a switch from a 2500 stall 11in. to a 3800 10 in. also have a 3.23 gear. 28 inch tire tho. you sure have a tall tire in that thing. might be time to put a lower gear in it.