1994 Mitsubishi Magna head replaced still whie smoke

Hi guys have a magna 94 with 2.6l engine

the cars was smoking orginaly when you first started the car. Give it a rev goes away then comes back

so I pulled off the head and seen the head was worn and corroded but head gasket looked ok no signs or traces or leaks.

i got another head from another magna that had done 160km didn't get it plained as I cheak it tolarance and was ok reseated the valves and installed new valve seals put engine backtogeather with new head gasket and is running

now when I first start the car no signs of smoke or anything even if I rev it but after 5 to 10min of idaling then I get white smoke coming from the exsaust

im stumped on what it can be maybe a sensor or somthing. If it was a gasket or crack it would start smoking straight away?

other thing is themostate is stuck open will this cause it.

any suggestions be great