KA lost power, temp gauge goes up at high rpms only

[SPIRONIUM]

1992 240sx ka24de motor 165k miles all stock. Threw the car in 1st the other day to get around some rush hour stop-n-go traffic, the acceleration went limp without warning. Since then the car has felt like it has lost all of its torque. The engine runs very smoothly, nothing seems abnormal at all, at highway speeds in normal rpms it feels fine. But when i try to push it to accelerate theres just no power anymore. I can barely spin wheels in 1st if i drop it at 5000, and its just sluggish to accelerate overall in high and low rpms. Seems to rev a bit slower in nuetral as well.

I also had noticed that if i push it to just over 5500rpms the temp gauge starts to shoot up! I let off immediatly, and the temp goes right back to normal in an instant. I let the car cool down to check my coolant system, everything seemed ok, so i hit the road again to see if it was just a fluke. The temp gauge goes up everytime once i hit that magic mark of 5800rpms. What is this a sign of if anything?, could it be that the gauge is just not reading correctly, maybe its just getting some static or some wire is arcing at high rpms?

So far i have checked compression ~155 all across with one high cyl at 162

Vaccuum is healthy, i tested it because i had reason to believe that the cat was plugged up, (i went a bit crazy with the copper gasket spray while putting the OE exhaust + cat on ). Even though vaccuum was fine i still deleted the cat and put on a 3" straight through header back exhaust. I was assuming that the power would go back to normal and then some because the suspected clogged cat was gone, but unfortunately i only felt a slight difference, car is still sluggish.

I replaced the engine temp sensor (not the one for the gauge), new NGK plugs, air filter, fuel filter, double checked coolant system for bubbles.

Mafs is fine.

What should i do at this point? I need some guidance. Thanks

[90nissan240sx]

Thats a hard 1... auto or stick? check engine light on or off?Maybe limp mode or somthing, set by the ecu. Do a fuel pressure test as well.

[SPIRONIUM]

Stickshift, no engine codes: ECU reads 55 a-ok. I'll check the fuel pressure soon as i get my hands on some kind of gauge

[boriquaS13]

Restricted radiator? When you checked your cooling system was it low on coolant? Any leaks at all?

[SPIRONIUM]

Coolant remains full, same with level on overflow, so there can't be any leaks. Is this a sign that a water passage is blocked, or could it just be that the water pump is going out? The second doesnt seem likely to me, I would think that the rubber seals in the water pump must go out way before the metal blade internals deteriorate.

I don't know, i believe that the active-at-high-rpms temp gauge and the power loss is part of the same problem but maybe its coincidence. Maybe i should stop looking for a connection and treat the problems seperately?

The problem isn't so bad that I HAVE to swap another KA right now, its 100% streetable the way it is. It is my daily driver most of the time but i do track it as well and i got a rallycross coming up in a few weeks. This is annoying..... Half of me wants the motor to fail catastrophically just so i can get over it and swap a new one. I'm gonna push it hard at the rally we'll see if it can take it in its current condition