94 firebird sluggish and makes popping sound

PLEASE help...my 94 firebird runs sluggish and makes a popping sound like back fire but, from front of the car. We have changed 2 o2 sensors and the spark plugs no change. It's like it boggs down then pops one or more times in each gear. Please help...any suggestions?

Sounds like the timing is off
cylinder is firing before the intake valve is completly in closed position (will cause back fire thru intake side of an engine)
Improper timing will cause an engine to run very sluggish
Or could be a burnt or sticking valve due to carbon build up

I had the same problem with my 91 Formula..turned out to be the coolant temp sensor, costs about $7 at Advance.