HELP! 92 gmc backfire through exaust
 

Hey there, I have a 92 gmc k1500 with the 350 tbi and a 5 speed. I have a wicked bad backfire/ lack of power only sometimes. While driving down the road normal rpm around 1200-1800rpm the truck will run fine while on the gas. Once I let off to say coast down a hill In gear it will sputter and the rpm will bounce up and down a little and then back fire wicked loud. After that it will coast awesome no poping nothing. Once back on the throttle and let off it will do it again. Also if I'm just crusing and I put it in neutral it will rev down to about 1500rpm do a little back fire then idle smoothly.

If some one knows what the issue is please help me. I have long tube headers with acell header plugs. It dosint smoke when it dose it or stall just back fire

Thanks for the help I hope I get some responses because I'm getting irritated with this