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Try to stay positive about it. There is a solution. Logic prevails and anger leads to broken parts or wasted money, right? It does for me anyway heh.
Anyway, you can also try unplugging the AMM when it's hot and see if that improves the performance. It could be failed and keep the engine running rich - which will work decently at lower temps and then make it run like crap when warm.
Also, what about the EGR?
Also, I would pull off each and every intake tube and reattach it. You could also have a crack in your intercooler or be missing the plug (to drain accumulated oil). It's found in the bottom driver's side.
You *might* have an intake hose collapsing once it gets hot, or an airbox thermostat that's failed in the "hot only" position (though my 90 doesn't even have the airbox thermostat, maybe they brought it back for the 900's).
If you think the cat is plugged, running seafoam through the engine should make a difference - either improve it or make it worse. The metal may be rusted and crumbling, but if it's blocked, at least part of the blockage should be soot(carbon) and whatnot.
Most importantly, I'd find out exactly what hose came off and what you reattached it to. Maybe it was supposed to be attached to something besides what you attached it to.
The dump valve (wastegate) usually connects straight off the turbo's output (cold) side with a very short (six inches or less) hose. If that's not working, you'll get too much boost, not too little. The CBV (which feeds the turbo output back into the turbo's intake) needs a vacuum line attached to it.
You can directly monitor the O2 sensor with a simple, cheapo analog multi/volt-meter (about $5-$10) on the 1.0V range. If it stays constantly to one side of .5V, you have a flakey mixture or a dead O2 sensor, though that probably won't cause the problem. If the O2 sensor is working, you'll be able to tell if the mixture is changing like it should - the voltage should sweep between 0.1V and 0.9V and should spend equal amounts of time on each side at idle, at steady non-idle rpm, on increasing rpms, and on decreasing rpms.
If it's a mixture problem, it should be really really obvious like the meter is pegged at 0.9V all the time. I believe that higher voltages indicate rich mixture.
Did you try the OBD1 test mode? I think it's mode 2.
Also, what does the turbo gauge do? Do you have one? If not, can you beg/borrow/steal/buy a vacuum gauge? You might have developed a leak in a metal portion, in the exhaust before the O2 sensor, in an intake or exhaust manifold....
Did anything weird happen just before it started acting up?
Did you check the resistance of the ballast resistors and of the coil? The power stage is always suspect in heat-related issues, but usually fails completely or works fine.
You can clean the ground plate that feeds multiple wires to ground on the fender wall under the hood.
Cheers! and Good Luck!
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