Chevy & GMC Vans

Just bought a 2002 Astro with 88000 miles. The previous owner had problems with coolant disappearing.
I figured it was an intake gasket, so I wasn't worried.
Topped off the coolant, drove maybe 2 miles, and in that time I noticed the temp gauge going sky high. The previous owner said it would do that, but go down. So I pulled over, and when I stopped, the gauge went down. I noticed that the heater began to throw heat then too.
Drove it home with no more problems. Drove it on and off around town, to see if it would start leaking, and for 3 weeks it didn't. Well I came outside one morning, and there was a puddle of antifreeze, so it was time to check out the intake for leaks.
I took the engine cover off, and noticed a wet spot in the rear of the intake.
I took it up town to do some errands, and the temp gauge went sky high. I didn't have any heat either. I pulled over shut it off, waited a few minutes, and re started. The temp went down, and I had heat. It did it a second time. and did the same thing.
So now I have the intake pulled, and the gaskets were warped.
Since I have it down, I need some advice on the weird coolant temp, and heater act up.
The previous owner said he put in a new thermostat, but I will replace it again.
Wanter pump is dry so no leaks there.
Would there be a major air bubble in the coolant lines doing this?
Please someone out there shed some light here. Ill fix anything it needs. But I need answers or advice.
All replies are greatly appreciated.

Thinking about it a bit, it almost sounds like the thermostat is in upside down, but I'm not sure if that is possible to do off the top of my head. If it's not inverted, it could be sticking OR the entirely wrong model (design temp) for the engine. Changing it out would be in your best interest since you already have the top of the engine gone and the coolant drained.

Water pump impeller could be on the way out, maybe. Was the blending valve for the heater core ever removed? I wonder if that could have been reinstalled incorrectly, also.

I'm leaning against air in the system as you would need to be really low to have a bubble get stuck in there over and over again and it doesn't sound like you were.

Do you have rear heat? Any issues with that? Have the vacuum hoses been replaced? The miles are a bit low for the year but the age is right for those to be leaking enough to cause problems. The vacuum is used to direct which vents are blowing air, based on switch position.

Once you have the gasket back in place, can you flush the hoses in both directions? Any gunk coming out with the flush?

That's all I've got in a 5 minute window of thinking.

The first thing I thought of was the heater control valve.

x2 on the heater control valve. I had the same problem for a couple days back in January ( engine would get to operating temp and hotter but no heat in the van ). I wish I could pinpoint to 1 particular item but in my case thinking it was the thermostat I tore it all down and replaced both the waterpump & thermostat. While in there I broke the 6 way heater control valve and noticed at that time the vacuum line controlling the valve had also rotted at the vacuum ball ( not allowing it to open and allow the coolant to flow ).