I hope Terry is right. If it's not the T-Stat, your water pump has self destructed. The M50 comes with a plastic impeller and in time, fly apart rather spectacularely, depositing bits of itself throughout the cooling system.
A T-stat is cheap, you might as well replace it. If you don't want to, pull it out of a cold motor, stick it into a pot of water and boil it. You should see it open (cylinder expand) as the temperature rises (close to boiling).
Also, overheating it this badly this often, I would perform a compression check. You could have warped the head or cracked the block. When you change the coolant (as you'll have to if you change the water pump, or top up if you change the T stat), check for oil in the coolant, and vice versa, when you do your next oil change, watch for white soapy crap in the oil.
Last edited by BigD; 03-26-2006 at 08:19 PM.