+10,000 what mgbman69 suggested. Cooling systems are the weak link on our cars. Best to go through and start with a completely fresh system. You'll be happy later down the road when you're not spending thousands of dollars to fix a blown head gasket. While you have things apart, might as well replace the oil filter housing gasket and o-rings, valve cover/spark plug gaskets, belts, hoses. Take a look at your bushings, ball joints, tie rods, tranny mounts and motor mounts. Rubber intake boots usually become dry and cracked which can cause problems as well. Definitely start with the cooling system though. Here's a really helpful thread:
http://www.318ti.org/forum/showthread.php?t=18066