When this problem started I was in the middle of chicago traffic, we all know how big city traffic can be at rush hour. I thought for sure the truck was going to die right there and I was going to be left in the middle of 355 waiting for a tow. I babbied it home down 355, to 55, to 80, to 39 to tonica exit 48 which is a 2 hour drive at the best of times. All the while cussing it and praying it would at least make it home for diagnosis and repair. It didn't send an engine code no idiot light on the dash, tried pulling the codes and nothing. Went out the next morning and fired the truck over, it cranked as usual until this horrendous clunk almost like the starter stuck (yet another problem with this truck) but it started. I drove the 9 miles to the grocery without problem, no hesitation, backfire or miss, but when returning home it started it again. keep in mind I let the truck warm for 7 - 10 minutes before driving on a cold start, and 3-5 on a warm start. I called the shop when I got home they said sure bring it in! (big suprise... not!) after approximately 6 hours worth of diagnosis, tracking, fiddling, and coffee breaks they still couldn't figure it out. At this point it became more than an annoyance, it downright peeved me expecially when the shop managaer said ok that will be 540$!!! I grabbed the owner and relayed to him what was going on and he dropped the charges to $75 for the use of the scanner and sniffer. Cool! I brought the truck home once again where it sat through 3 snow storms, ice storm, and rain until I read the other complaint of missfire on here. I checked around the local parts stores and found that the pre cat 02 was $70 and post cat was $80 ok... small price if it fixes it, if it doesn't then ok too because I don't believe the o2 have ever been replace they're probably close to being toast anyway. I grabbed my other truck went to town and picked up the parts and replaced the 02 pre & post. The start problem is still there but the backfire is gone :D

3.9 V6 start vehicle driving along about 50 to 60 kms, per hour engine backfire, seems like up through the dome, have to let off gas and step on it again to get it to go, seems like it going to stall, it will come out of it if i put the pedal to the floor, once warm up it doesn't seem to do it, very hard on gas. had it to the repair shop and they are stumped, thought they might of had it fixed but brought it home tonight and still doing it. they had said that a wire wasn't down in the distributor, hoping that, that was the problem wasn't. good thing they haven't done up the bill yet. any suggestions. look back at how bad is this problem.

UPDATE: To follow up on my truck's problem, thanks to the one that responded, it was the o2 sensors upstream, but I replaced both sensors, I got the check engine light to come on and store a code. I did this by disconnecting the battery one night, and next day truck acted up and by putting my foot to the floor, the light came on, Glad to see this, but that was the problem, all along thanks again.