You don't need a code reader for a 93 (it is OBD1). You just short the "B" and "C" pins in the 4 wire plug up by the battery and then turn the key to run and watch the CEL and count the flashes to determine the code(s).
If you haven't done a tune up (new plugs, wires, cap, rotor, air filter, adjust valves) lately, I would do it. Might also be a good idea to replace the fuel filter.
The EGR passages and EGR valve are bad about clogging up on these engines. So a thorough EGR system cleaning may be a good idea too.
You could also check/replace all the small vacuum lines too. A leaking vacuum line will cause an unstable idle.
The ECMs are known for going bad in these. I had one that would do something very similar to what you describe. It would run fine for awhile (or at idle), but would then bog out so bad that I would hardly be driveable (actually, I couldn't get much above about 25mph). The ECM was bad. Some can be fixed, some can't.