1992 LeSabre- High Coolant Temp

1992 LeSabre Limited- 146,000 miles

I have had this car for over 3 years, not having to add coolant. A few weeks ago, as I was checking the fluids, I noticed the coolant was low. Figuring it had developed a leak, I put a bottle of stop-leak in and topped off the rest with 50/50.

Since then, while driving, i have noticed the temperature gauge climb toward the red zone, and when it nearly touches the red mark, it drops down to normal operating temp by itself. When I see the temperature rising like this, I can also increase the throttle, and this will also send it back down to normal.

I figured this was a blockage in the system created by unnecessary stop-leak pellets. I decided to get a coolant flush at a nearby shop. As they performed the procedure, they seemed to use a straw-like "sucker" to drain the system, instead of using water pressure to "flush" it right out.

So the system has nice fresh coolant in it, but the problem still comes up while driving. I have also started to notice my upper radiator hose collapsed when I open the hood after driving (maybe I just didn't notice it before?).

I am looking to sell the car soon, and want to have the problem fixed before then. Does this sound like a stuck thermostat? Or something more serious like a water pump...?

Change the thermostat and see what happens.

Well, a friend of mine borrowed it, and decided to buy it.
I discussed the issue with him, but he says it never did that while he drove it. Having to leave for work early in the morning, he has been using the heater, and he says that might have taken care of the problem before he got a chance to see it.

Thoughts?

I havent used the heater since before this all this came up. Did the fluid just need to circulate through?
...Or is the use of the heater keeping the system cool enough not to register a high temp?