Engine will not heat up in winter, overheats in summer.

Engine will not heat up in winter, overheats in summer.

Hey all, I drive a '94 Audi 100. In the dead of winter, I can start my car, and as long as it is sitting idling the engine will SLOWLY warm up--but rarely to full running temp. Once I start driving, engine temp quickly falls to near cold. Heat blows when engine gauge is warm (leading me to believe it's not a heater-core clog). Side note: Cardboard in front of my radiator seems to help it stay warm for a bit longer.

In summer, car will struggle to stay at running temp and want to overheat. When it starts overheating, heat blows VERY hot.

If it was just one or the other, I would pin it at bad T-Stat in a minute, but since it is both I am stumped. Don't think heater-core is clogged as stated since heat does blow eventually. Come to think of it I don't remember the last time I heard my fans kick on though. Clogged radiator? But this wouldn't explain my cold engine in winter. Possible weird T-Stat? I hope not as a T-Stat in an Audi 100 is a pain to get at. Side note: My oil pressure gauge likes to max out while motor is under power--no known cause. Thanks in advance!