1991 Jetta cooling problem

I'm having a serious cooling problem with my 1991 Jetta. First I'll give you a little history. I bought this car from a co-worker. He drove it about five miles one way to work. I drive fifty miles one way. I drove the car for about three weeks with no problems at all. One day I was driving home from work and it over heated. I pulled over immediately let the car cool put water in resivor and once again it started overheating almost immediately. I towed it home replaced thermostat still ran hot, replaced waterpump still ran hot, had the heads worked still ran hot, replaced all hoses still ran hot, had radiator checked, proved to be good, ran water through engine & heater core, water circulated didn't seem to be blocked. I once drained the engine mixed my coolant 50/50 filled system with coolant according to directions in manaul. When I started the car the top hose got hot just as the manual said it would if the water was circulating. The temp gauge never went above 1/4. I drove the car all day with no problems at all. I got home put front brakes on the car took it out for a test drive and once again within ten minutes it was running hot. It's definitely running hot it's not defective gauges. In all I've put three thermostats, two water pumps and replaced all hoses.
I'm at a total loss. Any input would be appreciated.

Is the electric fan coming on?

Is the electric fan coming on?
I'm not really sure. I wired the fan direct so it would stay on while I was driving. It still ran hot. When should the fan come on on it's own?

What do you mean by "heads worked"? Have you noticed excessive steam in the exhaust or done a compression test? Does the lower hose & radiator get hot & have you felt for cold spots on the radiator?

I had the heads looked at. The machine shop said that they were slightly warped but not bad. I had them resurfaced and they also checked for leaks. All checked out good. Since I put the heads back on I have noticed some steam coming out the exhaust, I don't think it's excessive but there is definitely some coming out.I havn't done a compression test. I'm not sure if the lower hose is getting hot (I haven't messed with it in a couple of weeks, I don't remember.) but I think it was. But I don't think that the radiator is getting hot. I do know that the top hose only got hot on the day that it ran without overheating. I also noticed that the hoses to the heater core doesn't get hot when it's over heating but they did when it was running cool. I've checked the oil and havn't noticed any water. Do you think I should try running it without a thermostat or maybe bypass the heater core and see what happens?
Thanks for your input.

Try this; release any cooling system pressure by loosensing the cap, tighten the cap back. Start the car & rev for about 15 seconds then open the cap & see if it built up pressure. There should be only very little pressure on the system or suspect the head gasket or a crack.

boschmann, thanks for your help. I finally got it going. It ended up being a faulty heater core. Once again I may need your assistance. The same day I got it going it quit on me again. I had driven the car most of the day and later that evening it wouldn't start. I'm not getting fire from the coil. Is there a way I can check the coil? Or can you reccomend the steps I need to follow to trouble shoot? Thanks again for your help with the cooloing situation.

Pull the distributor harness & see if you get 12V between the outer two pins on the connector (key in on position). If so then replug the connector & backprobe the center pin with a LED test light connected to battery positive, crank the engine & the light should flash.

I don't mean to sound stupid but I'm not sure what you mean by backprobe. I have a voltage tester that looks like a small screwdriver with a pointed tip and a lighted handle. Is that the type tester that you're talking about?

If it has a light bulb then no, it needs to be a LED light. Backprobing just means the connector is connected & you have to pierce the wire at the connector or insert the test probe into the backside of the connector to get the reading.