[amac209]

1999 has no cap or rotor. there should be misfire codes if it's misfiriring. look for a a vacuum leak around the plastic intake plenum they have a tendency to warp. i assume your mechanic checked the egr, tps, iac, map, O2 , maf, iat and all the other sensors. what happens with the vacuum leak is vacuum is high at part throttle so air bypasses the maf. have you replaced your plugs and fuel filter recently?? if not do it now they may or may not be contributing to the problem but are part of the whole preventitive maintenence deal. get your buddy to have another try with his scan tool just because the CEL isn't on doesn't mean you don't have codes. good luck. oh right one more thing to check for is carbon deposits on the throttle valve and body. good luck!!