I have a 97 with the 3.0 pushrod engine. The car runs fine most of the time then one day it will spit and cough under load...the next day it is fine. Pulled the plugs...looked good, replaced anyway...checked compression 150psi on all 6, replaced the plug wires, fuel filter, and cleaned the maf. All this looked good. Car still backfires or spits(through intake or exhaust can't tell probably intake) either cold or warm at all rpms except idle when under load. feels like a 4 cyl and is not setting any codes. Someone suggested clogged egr ports. The egr valve itself holds vacumn. Any ideas??