The unfixable mystery problem

[Stormin]

I have a 1989 turbo with recent mild modifications, a slow work in progress. The car backfired badly a few weeks ago at speed, the check engine light came on and I had similar symptoms, would not rev above 4,000 rpms. Checked everything, changed TPS, etc., no luck. Finally found a simple cost free problem. The lower intercooler hose had come off, huge vacuum or pressure leak. Re-attached it and made sure it was secure, problem solved. One possibility. Hope this helps.