I only just brought it v6 1991 wont rev out clean dies down at high revs

If you hit the Rev Limiter on a Ford, it will simply bounce the rev back down, to protect the Engine. So, no high Revs. But, you should be able to bring it slowly to a High RPM. That is normal. If there are no Check Engine Light on. You are most likely hitting the rev Limit and need to stop.

As long as the idle air control valve is working, ( I assume this is what you replaced, I would first check to make sure it is operating correctly), If it is, then Id begin searching for a vacuum leak

check fuel pump pressure first ,

clean your High Idle control valve. It's attached to the Throttle body

I would replace the IAC air bypass valve, also I would have the fuel injectors cleaning service done by a dealer, not the pour in the tank cleaner, it doesn't work.

Still think ot vacuum related .This will push the revs up sky high .Get a good cigar and pulloff a vacum pipe .Take a good puff on the cigar now puff the emoke down the vaccum pipe .Now do you know what your looking for .You got it ,,, smoke ,,,,than you have found the leak.Thats if you have one ..Can you get a obd check it will tell you what it is ..Thermostat removal wont change the engine revs

I have jus brought a golf 1.4 petrol it was fine full serivce history good condition just i found out when i rev my car n let go it drops very fast like it will die out n kiks in again do yu know what is wrong with my car

Does it have a Mass air flow sensor? If yes, unplug it and see how it acts.

You need to have a tech II plugged in to see which sensor is playing up .

my friend has same model its normal he said it does it more so when its cold