This might sound Silly, but it happened to me. I had the exact same problem. When I changed my MAF, My car drove fine,idled fine, everything checked out fine. But at WOT it bucked and sputtered like a mule.My problem was due to fact that the MAF plug wasn't seating good to the new MAF. It just wouldn't stay tite. Check it out, it might be as simple as that. I was so relieved. I put a zip tie on the pigtail real tite and then snapped in on. GOOD LUCK!!
'88 GT vert 519rwhp/450rwtq. Systemax intake ,Edelbrock Performer RPMs,TFStage 1, Novi@12psi, UPR Powerpipe, AFM Mr.Freeze, 60's, ProM 80, Aeromotive 340lph, FRPP headers, UPR Offroad X, Dynomax exhaust, aluminum driveshaft, AFM Competition Stage 4 clutch,3.73's