2000 Grand Prix GT - no power

Have a GT 3.8L -no s/c with 224000 miles. Car is great however in the last month it has been getting steadily worse. Engine has been running rough & shaking under acceleration. Now after a couple of miles the TEMP light comes on, and it feels like something lets go in the engine, it has no power and idles poorly.
Could this be a cat converter issue (blockage). It has had full tuneup (new plugs, wires & coil packs have been checked).
After cool down it runs well for a few miles and then repeats.
Please advise

Is the motor overheating?, and check your Idle Air Controller, i had trouble with my idle and i cleaned it up and it was fine.

I'd take a hard look at the cooling system...have you ever refreshed the Dexcool coolant?

While it's apart I think you have to look at blockages in the radiator and heater core...use a garden hose to flush thru these and check the presence of absence of flow.

I would change all the soft hoses as a planned maintenance item and also the thermostat if it was my car.

have the UIM/LIM gaskets been changed? this may be your problem...

Let's get some more info before speculating any further.

1. Is your check engine light on? If so, have you scanned it to pull the codes (post the results if the answer is yes)?
2. Are you mysteriously losing coolant?
3. Does the temp light come on after a couple of miles of driving even if the car isn't warmed up?

Had cat converter changed. TEMP light no longer comes on. Engine (car) still shakes under power. Now engine revs up to 2000 - 3000 it does settle down after awhile. Check engine light has not come on. Engine coolant levels are fine have not lost any fluid. Engine does not overheat. Just runs like crap>

unplug the maf and start it up.

Also is it just coincidence that you replaced the cat and now the temp light stays off? seems weird.....

Vacuum hoses can cause rough running also.

As Tblake suggested, unplug the MAF and see if it runs better. If it does, you need a MAF sensor.

Ok guys, thank you for your help. Finally got car to give up a DTC P0118. Changed the ECT (electronic coolant sensor) $40.00. Problem solved.
It was also a partially plugged Cat Converter that I changed.
Faulty ECT effects fuel injection causing engine to run rough.