[cadgear]

You -are- due for a coolant flush, regardless of its condition, but sludgy is common of old coolant. You'll find that true in either green or dex-cool systems; old coolant is sludgy, muddy, gel-like, etc. HOWEVER. Check when the engine's hot and the hood up; is there a strong smell of coolant? Check the oil just to be safe: if its brownish like chocolate milk/pudding, or gravy, then you might have a blown intake, like Blake said.

Best way to verify this is clean out the resivoir (take it off the car if you need to), drain the system by either taking off the lower radiator hose or, if you want "the right way", there's a drain cock on the lower driver's side that unsnaps with a 1/4" socket extension. Once you drain it then replace the lower hose, pop off the upper hose, point it downwardish, and stick a water hose in the radiator fill. Crank it up and let it run a few cycles of opening and closing the tstat. I know, that's not the GM Specified Certified By The Book way of flushing, but it works just fine for me.

Refill the system, bleed the air (pretty critical), and check it in a week or so. If its down low again, you know you've got a problem.