90 Sunbird le acting strange.

Hello, my 1990 Pontiac Sunbird le w/ 2.0l OHC has been acting strange the last three days. This is my work car. And I go to work at 2:30 am and its pretty cold out. The car starts fine, runs a little rough. I'm pretty sure thats a vac leak. But just recently the car has been struggling to get me home. When I'm done delivering my payload 2 hours later, the car acts as if I were pulling a boat and spits and spudders a little, Idle is irattic, and I cant keep any speed in third gear. Let me make it a little easier for ya. I'm done with work wohoo! I'm going home. The car is running ok, and goes through 1st no problems, 2nd a little spit and spudder and jerk then that stops. The car gets to 45-50 and hits 3rd, and no mater how hard I stomp the gas it wont gain any speed just quickly slows the car down (as if I were towing a boat) to 35-40 and then the car down shifts to 2nd very hard. Once the car is back in 2nd I can gain speed again to 45-50 before the car shifts to 3rd and I repeat the proses the whole way home. I kinda think my engine is overheating but my gage never goes over the half way mark. I pulled over last night while the car was acting this way, popped the hood to see if there is a lose wire somewhere and my exhaust manifold was red hod (engine cab glowing). Also I live 11 miles up a mountain. I gave the car a complete tune-up (plugs, wires, cap, fluids, filters, etc) about 2 months ago when I got the car.



Thanks for any info.