1993 Geo Storm Car Loses power when engine warms up

I have a 1993 Geo Storm standard, I bought this car for $400 and have had no problems except for this; when I start the car up it runs fine and has plenty of power, then after about 30 minutes when the engine warms up, I lose all power, car still runs but needs to be brought to around 5000 RPMs before shifting because the power is that bad, almost as if the spark plugs were bad. The only thing I could think of was the coolent temp sensor, so I replaced it and saw no change.

Thanks!
-Mike