Some questions. Does the amount of fuel in the tank have any bearing on when it starts missing? Maybe the fuel is rushing away from the pick-up & starts to draw air. Does it have dual tanks & does it matter what tanks its on? As far as checking obd1, I can tell you how to draw codes w/a jumper wire. Under the hood at the L hood hinge area is where the obd connector is. There usually in a protective cap. One is a single wire (STI) the other has 5 or 6 wires in it.
1. Warm the engine up a bit then shut it off.
2. Take a jumper wire, connect one end to the single wire connector.
3. Looking at the 5 of 6 wire connector face you'll see 4 wire slots that look like this - --- (forget these). Above that the connector is angled on each end w/2 wire slots. Connect the other end of the jumper wire to the SHORT angle side (STO).
4. Get in the cab, turn the key to ON and watch for the ck engine light to flash. Count the flashes. Should be 3 digit sys. Example. 2 flashes, pause,
3 flashes, pause, 4 flashes pause. Code 234 EGR sys. If ther are more codes than 1 it will then flash the next code.
5. After this set of codes you will get 1 flash w/a long pause. It will then flash more 3 digit codes. Write these down also. It will repeat the whole process twice.
6. One last thing. Don't disconnect the jumper wire w/the key on. That will erase the memory codes. The memory codes are the ones after the 1 flash and the long pause. As long as you don't disconnect the jumper wire with the key on it will keep flashing the code sequence. Just turn the key off a few seconds, then back on and the process will start again.
A little wordy, but that is as simple as it gets. If you decide to try this, post the codes here before you start buying parts. I can tell you which to ignore regarding the problem.