I have a 93 sidekick which has had a new motor just recently installed from a 2002 car.

It has just started over heating the last couple of days so I have installed a new thermostat hoping this would rectify the problem but it hasn't. I can drive maybe 5 min until steam starts coming out from under the hood and rad fluid starts leaking out. The idle is also jumping up and down while sitting idle. It normally idles just above 1000rpm and it will jump up to about 2000rpm and then keeps going back and forth. not sure if this is related to the over heating problem? Any advice on what to check next?

Thanks in advance.