1G '91 AWD won't rev beyond 5500 rpm

[quickduc]

This problem has been ongoing for years.
Plugs and wires don't help. Swapped the coil from my spare engine, no change. Ignitor box, ditto. Cleaned the injectors. (removed them and soaked them in Sea Foam with a power supply to hold them open, to expose the internals for complete cleaning. Wow! What a difference in throttle response and overall performance. Like a new engine! Still won't rev up.)
I'm not seeing/hearing/feeling any boost leaks. Have not installed a real boost gauge. Just has the oem dash gauge.
Replaced the fuel pump when it started stalling below a 1/4 tank. Put in a Walbro 255?
Except for the pump, the car is stock.
This problem aggravated my father the PO, and his mechanic could never figure it out. The car would run 140mph originally. Dad had a bunch of these DSMs, but this one had him stumped.
This has been an erratic problem. When I first got it ~'03, it occasionally wouldn't rev up. Now, it is reversed, and mostly won't. Runs strong until 5500. It acts like it has a rev limiter.

Has that CARB mandated generic cat that strangles the motor, but this problem predates that by a decade or more. (I suspect that CARB picked the 420 cat for all the DSMs, as I estimated the matrix is about 63% volume of the OEM cat. Strangles that poor motor. I can understand why people take the damn things off with this sort of idiocy. Seems to hurt fuel mileage too.)
Got it with 100k miles, now ~168k.

 

[Charliethatguy]

Does the car have any kind of logging software (Link?)

 

[SouthIndy_Talon]

Does the car have any kind of logging software (Link?)

No... minus the exhaust, the car is 100% stock at the moment

 

[quickduc]

Try adding the hoseclamps in the middle. I had a pile of large ones on hand, but two radiator sized hooked together should be long enough to get around it. It seems to give it some structural rigidity to keep from collapsing. Ideally they should be on the inside, but that's not very practical. Gluing some metal rings or sleeves inside would work, though. Finding the right size/shape would be the problem. I only bothered to put one clamp on, but multiple clamps would work better. I was in a hurry.