I have a 99 Ford Ranger 3.0, flex fuel, 6 cylinder. It keeps overheating to the point of blowing off the reservoir tank lid. I have replaced the radiator, thermostat 3 times, and the water pump. The compression is good on it so far. I start it up and let it sit for a while until the temp. goes up on it to where it starts to make some heat. I run it and within 5 minutes it over heats and I can hear the lid from the reservoir tank blowing off. The temp will go clear down to cold and slowly go back up to over heating then back down to around the normal range. The heat isn't quite as warm as what it was before this all started happening. Is there anyone out there with any suggestion.

 

Stop replacing things................

Cold engine
Fill rad to top and leave the cap off
Start engine
Watch for coolant coming out of rad cap opening, it shouldn't.

Let it idle, if coolant continues to come out rad cap opening, some times in burps, then you probably have a cracked head or head gasket leak.
This can mean coolant in the oil and white exhaust smoke but not until it gets very bad, before that it just causes overheating.

A cold cooling system has no internal pressure, a water pump is not really a "pump" it is a circulator, it doesn't add pressure.
A cylinder in the engine has 170psi pressure when cranking and 1,000+ psi when running/firing.
If even a small leak is there then some of that pressure is passed to the cooling system, this displaces the coolant in the engine with "air" from that cylinder, and it will push some out the rad cap opening if cap is removed.
If you leave the cap in place then it will hold that pressure in until it gets above 16-18psi, cap rating, and then coolant will be pushed out into overflow.
When enough coolant is lost the engine will start overheating, t-stat will open quickly and cold rad coolant will rush in making temp gauge dive.
The "air" at the top of the engine will now be able to go to the top of the rad because t-stat is open and that will start the overflow tank bubbling, it isn't boiling, it is just "air" being purged.
Gauge will return to normal with the rush of cold coolant but the leak will continue to pump "air" into the cooling system and displace the coolant, engine will start to overheat again.

Sound familiar?

That's the bad news, if it is a cylinder leak, search for glove test at our site, I have posted it a few times, it is an easy and quick way to determine if you have a cylinder leak and which cylinder it is.

Good news is there "might be" another reason.
When t-stat is closed the water pump needs a by-pass to circulate coolant, this is the heater hoses on some engines, it varies by year on Rangers, but most have this.
Follow the heater hoses, if they go directly to the firewall then the heater core is the by-pass, and if the core is clogged then engine temp can go up and down randomly, but NOT over heat, just a little above 1/2 then it will drop below 1/2.

If you have a 4 connection heater hose assembly then that is the by-pass.
Mark or take a picture of which hose goes where, because you WILL forget, lol, and then remove the 2 that go to the engine and put a connector between those 2 hoses with clamps, so by-pass the "by-pass".
Then check for over heating
Extreme long shot from your description