sounds like a misfire or plug wires routed to the wrong fireorder. raw fuel dumped into the exaust and boom! the fireing orfer should be in the intake, check your timing to.

sounds like the firing order is wrong, misfire or timing is off. fuel is dumping into the exaust and collecting in the muffler.

Fire order is 123456, I would figure after a shop had it at least something that easy would be correct.

Wouldn't you think the shops checked timing and all. I got a miss I can't find. Tell us when you get an answer

Hey Show where is your fuel filter located if I may ask.

Have you checked the condition of the fuel injectors?

you would think.. could be a crack in the cap vacumm advance not functioning.
Sean