This could also be related to a bad n75 valve. See this picture? It's right above the coolant tank w/red hose coming out the top and black hoses on the sides. It controls the turbo boost. If it gets stuck or loses air pressure, it will not work right.
From your description, it sounds like you might be in limp mode all the time. Your car should be able to go faster than 85 mph without a problem. I don't know the top speed, but an unmodified car should be able to get to at least 100mph I'm guessing. Not sure exactly,but definitely faster than 85 mph.
It's the lack of power that causes you to slow down while going up a small hill. Your car should have about 90 hp stock, if you are in limp mode, you only have about 50 hp.
Because of the mileage of the car, I would also suggest replacing all the vacuum hoses that look even slightly worn. These are the colored or black 3-4mm hoses with braided covers. If they crack and leak due to age the car will run, but you will lose power.
Lastly, you need to figure out what the codes are, if any, on your car. Take it to autozone and they will scan it for free. Post it here and it will tell you what the possible problem is.