backfiring 454

I have an 88 sub 3/4 ton 454 2wd 400 turbo when i tow on the highway it will start to backfire though throttle body does this for 10 to 15 seconds then stops truck runs fine again . then ten to fifteen miles down the road it will do it again . it really makes me and my wife nervous be cause we start to think its gonna leave us stranded along side the road with kids and camper waiting for a tow truck. it has never died just backfires though throttlebody. I have hooked up code reader and it comes back with no fault codes. took it to local shop and 200 bucks and a week at the shop they said we dont know . Even hooked up camper and took mechanic for a drive . he just got an extended break cause it didnt do it. If anyone has any ideas I would love to hear them . love my truck and dont want to lose it. wife says she doesnt trust it, and wants to get rid of it . so please help me keep my truck

when is the last time you did a full tune-up?
cap, rotor, plugs/wires,fuel filter,air filter,pcv valve ect.

plugs,wires,cap,rotor,and air filter,all changed last summer. but this truck is only driven about 3000 or less miles a year and was backfiring though throttlebody before this was done. only does this when towing

Try this.
There is an EGR valve to the left of the throtel body. Unplug the vacume hose, plug both ends . A Plug in the vacume line and a cap on the EGR's stem.
This isolates that valve.
I have an 88 gmc and now that I am thinking about it I had that simular problem 10 or so years ago.
Disconected the EGR and the problem was gone.
Mabey give it a try.
It wont hurt the engine to run with the EGR disconnected temporaraly.

ok i'll give that a try .thanks

Pull the distributor and check the condition of the gears and bushings.

I changed fuel filter again runs great again just got back from 200 mile trip towing a camper , not a single problem so thanx to all that replied, i read many threads on this site and found one that described the same symptems my truck had they called it lean popping . i guess from low fuel flow though a dirty filter.