1999 Silhouette flucuating heat gauge
 

Hello! Hope someone can help. I am at my wits end here.
I have a 1999 Olds silhouette.
The van was sitting for 5 years before I took possession. And it was leaking from the water pump gasket. So I changed the water pump.
After a 200 mile trip, with no warning, the heat gauge shot up in to the red. I immediately pulled over and opened the hood and steam started pouring out. I immediately thought stuck thermostat as the upper rad hose was stiff, etc. So replaced thermostat.
Shortly after, the coolant light started flashing even though the coolant was full. Otherwise it ran fine for a while and had heat.
Then the heat gauge started going crazy up and down. Didn't matter if it was in idle, driving through city or highway. It didn't overheat again, but the gauge would go over the halfway mark and then drop far below normal and continue this behaviour.
The heater would only work sometimes and only when the temp gauge was low.
I assumed this was vapor lock so I bled the crap out of it. Parked on a hill, opened bleeders, opened rad cap, installed bleeder on hose that runs to upper intake manifold and bled through that, etc.
It didn't help much so I flushed the system like crazy and replaced the rad cap. Didn't help.
What did help some was pinching off the hoses to the rear heater and then, later, not using the heater at all.
I have been able to drive it by bleeding when the temp reaches the halfway mark then it goes back down and I drive on.
I did a block test and the solution did NOT change colors however when running the engine with the rad cap off the antifreeze would spill over.
I changed the thermostat 2 more times to make sure it was not faulty, but it did the same thing when I drove it without the thermostat in it.
The oil (and other fluids) look fine. Even if the antifreeze does look a bit dirty. No white smoke. No visible leaks. I can't tellfor sure about an internal leak because of how many times I have bled it and had to top off from losing antifreeze that way. I put a piece of cardboard underneath it and could not detect any leaks.
Another thing I noticed was antifreeze would overflow when in idle with rad cap off, but would be sucked back into the radiator when revved.
I took it to a mechanic and they said i needed a new water pump and thermostat, which is obviously not the case.
Any ideas?