car overheats after 20 minutes-- engine runs good and has a lot of power

 

Not good. When you say overheat. Meaning the temp gauge just climbs up to red rapidly or slightly?

Also, were you using the AC??

 

Also, Is it happening on freeway or street driving or both? Have you checked the fan?

 

it takes about 20 min to get to 1/2 way on temp and after that it climbs to super hot, fan come on first

 

On my 2006 XJ8 VP one of the ears inside the thermostat housing broke off allowing thermostat parts to fall into neck of crossover restricting water circulation. The radiator cap did not relive pressure (when I tested it relived at 35 psi) extra pressure spilt (fiber ) crossover pipe replaced as an assemble and New radiator cap 10K miles ago March of this year. Works like new

I was able to see water spray when I used a radiator pressure tester Just don't exceed 15 PSI

Good luck

 

Can you get heat from the interior vents with the heat turned on?

When you are in overheating mode turning on the heat high can help lower the engine temp in an "emergency". Not at all good to be running the engine when temp is high. Also the idiot gauge registers high long after the fact.

 

Is this in the highway or street or both? Did it happen when ac on or also off?
Having these answers will help pin point the problem.
Either way replace the thermostat and go from there.