Start car in gear to bypass 3K idle
It starts and idles at below 1K

As the car warms up the idle gradually increases until it begins to bounce between 1500 & 1200 rpm when fully warmed up.

Any ideas please

Thank-you.

 

Vac leak somewhere that's getting larger when heat expands the area at the leak. 1st guess, could be other things...

 

My vote goes for Wayne's theory. Look for missing gaskets or cracked gaskets.