99sl2 runs rough at speeds over 50mph
 

Car idles fine, revs fine when clutch in, drives fine at city speeds. On highway at speeds over 50mph, slight miss, runs little rough, not enough power to maintain speed going up hills, or into wind when cruise set at 70plus mph. I've already tried spraying throttle body cleaner and replacing throttle position sensor. I'm wondering if possible cat converter. I tried getting code, but, unable to pull code due to corrosion on computer plug in. I did notice that after replacing throttle position sensor, check engine light did not come on. (Light was reset due to unplugging battery.) Any ideas?

My first thought is when was your last tune up and second was clogging cat con. Reach under the car with a rubber mallet and hit the cat con and listen for crap bouncing around in there.

Just went through this a couple months back on a 95 SL1 automatic -- so some differences -- and it was the plug wires. New wires and plugs [was time anyway] and the rough running/missing went away. In my case I measured the wires once and they read good. After not being able to resolve the problem otherwise I went back and checked again. This time I noticed that if I bent the wires while measuring them a couple of them would go "open".

It could be a plugged cat or a weak fuel pump or clogged fuel filter.

If you have a scan tool that will read the codes on a Saturn my ideas are as follows.
If corrosion of connector is a problem, guessing what might be broken is little better than a shot in the dark. "It could be" a lot of things and very well might be. 'So go about cleaning the corrosion. White Vinegar will help dissolve some crap, WD 40 or it's equivalent can help as can an old tooth brush to scrub with.
If you think fuel pressure is a problem buy a gauge to measure it with. I have found a fuel pressure gauge is ONE of the most important diagnostic tools to be used with a fuel injection.
Buy a manual, Chilton, Motors, Mitchel's or what ever. It beats guessing and being wrong.
I have had catalytic converters go out. They plug themselves up. While banging on them might indicate a problem, but one that has shown up in every case was the symptom of the engine running hot when pushed hard and a measurable lack of performance. And when you start the car it sounds like some one threw a hand full of gravel at the car and hit a heat shield under the car when doing so;
The only way I know for sure to check it is to unbolt it and look. Of course rust might not make that very practical.