1999 Ford Taurus

I have a 1999 ford taurus I changed the head gasket, and after I put everytning back together and started it up, it makes a popping sound out of the intake. I have taken i9 a part about 5 times and tried adjusting the valves because I thought I wasnt doing it right, but I know it is right now for sure, but it still pops, im pulling my hair out my wife is furious. Does anyone know what could be causing this. Could bad gas cause this maybe?
June 24, 2012.

You might have missed a tooth on the timing belt and it is off timing a little bit or the camshaft sensor is off calibration from the air gap being disturbed.
I am going to give you a general diagnostic walkk through for throttle body back fires to eliminate some possibilites as the process of elimination is going to be the best way you wil have to find the problem. You might see something in the diagnostic that makes sense as being the problem after the head gasket was changed. Just us e the link below;

When I unplug the #3 cylinder the popping goes away.

Jul 20, 2012.
Is that cylinder within the pressure differential specification. Usually around 20% of highest cylinder pressure for the lowest pressure.