Overheating

I need some advice. I have a 94 Infinity J30, 170k. It started overheating so I replaced the thermostat. That didn't help, and it overheats faster everyday. Turning on the heater worked at first, but now it only slightly slows the process. Also, the car only takes about 15-20 mins. to cool down to 1/4 warm from all the way hot. The water pump makes no noises, and the radiator has no leaks, and the hoses in and out of the rad. are new. However, when changing the thermostat, there was a fairly good-size chunk of pink slate-flake-like rock behind it. Further inspection found the housing for the top hose (not to the thermostat on this car) had nearly a quarter inch of this sediment cemented to the bottom. And the radiator has almost a half inch of what looks and feels like dirty Crisco. However, there is no sign of water in my oil. My question is: Do I just need to flush this? Or is this problem a lot bigger than my paycheck?

Head gasket blown

Actually, I found out it was the radiator. Somebody put so much stop-leak in it, the little slots inside were as thin as my fingernail.

This is good. It's a lot cheaper fix.

Keep in mind that someone put stop-leak in it for a reason...
A good cooling system flush would do it the world of good... pull off one of your heater core hoses and get a fire reel hose into your radiator hoses. Get some real pressure in there to clean out your motor and heater core... that way the prob won't come back...