Vw corrado 1987 1800cc 16valve

i recently bought my first corrado, the first day of driving it it overheated but only after 30miles! i let it cool then replaced with water. got it home nice and steady and was ok stayed at 95-100 degrees, once home disconnected the bottom hose and flushed the system and once again ran ok for about 20 or so miles put my foot down then overheated again, when i say overheated i mean doing a great wee on the floor it was bouncing hot, thought about a low temp therm switch, oh and fan comes on once i turn the engine off so bit too late really, please help!!!!