Entire electrical system is intermittent.

I can't explain this, which is why I'm hoping someone here can...

My car is a 1988 Sundance ((2.2 Auto)) that I've had for over 7 years now. It's never really given me any big problems, but now I seem to be having this huge one. Electrical systems are an absolute nightmare for my comprehension.

When I first got the car running again, after a swap (bad block, swapped in identical engine) it ran perfect for about 2 weeks. Then, I started noticing the temperature gauge needle would start dancing around, fluctuating between 5-8 lines on the gauge. I just prayed it wasn't a faulty sensor, and just averaged the temperature of the car to be in the middle of the range the needle was moving. Then, about 3 weeks after that, the tachometer started doing the same thing. That kinda worried me, I didn't know what was happening now. I just ignored that. Then, that night I was driving home and everything on the car was going intermittent. Headlights, gauges, interior lights, fuel pump, spark plugs etc. Everything. But, I barely made it home. The next morning I went out there, and tried to diagnose it, but when it started immediately, I just decided to take it around the block, and then it seemed fine. (except tach/temp going ballistic) And it WAS fine, for the last 2 months. Until tonight. '

I was driving home on the highway tonight, and the same thing started happening, all lights, fuel, spark, etc were going on and off and on and off and the whole car was spazzing. Well eventually it crapped out and there I was, stuck on the highway around midnight. Now, whenever I would go to start the car after that, the starter would turn on no problem, it seemed to be the only thing that wasn't intermittent, but the car would crank and crank, and nothing would happen. I went out and jiggled and cleaned the battery terminals, nothin changed. So i shut off the lights, let the car sit completely off for about 10 seconds, then tried again and it barely sputtered to life. I drove the rest of the way home not going under 3kRPM just to help the alternator keep the car going until I could get home. When I got home, parked on the wrong side of the street, I went inside and then said, let me go try to put my car on the correct side of the road. Went out there, no lights on, started up very rough, and was able to turn it around to park it correctly. Then, while it was running, i turned on my headlights, and the car immediately sputtered terribly and died about 3 seconds later.

My first guess would be "Duh, the battery / terminals". I can't understand this, because this battery and the terminals are 4 months old and are clean as a whistle, and when I was on the highway tonight, I cleaned them again and there was no change. I'm going to pick up a new battery in the morning because I need one for another car anyways. But this is killing me because the Sundance is my daily driver and my show car, and I cannot figure out this problem for the life of me. So help me out, have you had it happen to you? What am I doing wrong?

Im also going to post this in the "Need Help" forum because I need as many answers as quickly as possible. Thanks for reading this essay.

If it were my car I'd start with the basics, Power and ground.
You'll need a wiring diagram and a volt meter.
Bust out the handy volt meter and start checking things and writing them down. You'll want things written down in case this is a long term issue. You'll be able to revisit your previous findings.
DO NOT THROW PARTS AT AN ELECTRICAL ISSUE....unless everything tests ok and can't test the suspected bad part like the main power driver board
The fact that you're having issues with multiple systems leads me to believe that it's an ignition switch problem or a main power issue ( either power module or bad grounds...lots of wires could have been tugged / moved when you were swapping the motor )
The ign switch can be "toned out" to make sure it's connecting where it's supposed to connect.
If you don't know what toning out a wire is, google it, sorry but I'm not going to get into "how to use a DVM/DMM" and if you can swap a motor, a digital volt meter is a piece of cake :)