overheating prob. 83 733i

I am new to this forum, but am looking for some advise.
I just picked up an 83 733i and the only problem is after maybe 10 minutes driving it gets really hot!
No aparent leaks, nothing in the exhaust or oil......but it gets so hot that the pressure releases out the overflow tank.
Maybe Im just missing something, but any insight would be greatly helpful.

thanks

if i was you id check the rad cap 1st dont hold me to this but im sure there was a call back for bmw,s for caps might have been 3 series if that is fine check the thermostat and the fiscus fan common on bmw,s

Water pump.