The trigger contacts in the distributor fire three injectors at a time. If one is bad the engine will run only three cylinders. The trottle switch can also be a pain particularly if someone had taken it apart and didn't know what they were doing. Another item that can cause all kind of grief is the vacumm sensor. The vacumm sensor has an adjustment on the backend of it(this may be behind epoxy). Once exposed adjustments can be made to the vacuum sensor and will affect how the engine runs. If this isn't working right the engine won't run worth a damn.

Make sure all the wires to sensors are making good contact. The thermal time switch(has two wires on it) located near the front of the engine. Disconnect the wires the the engine won't run below about 5000. If you have an injector that is not closing properly that will also cause the engine to run terrible but you will seen black smoke coming out of the tailpipe.

Well I hope this gives you some areas to check out and hopefully help you with your problem.