Cooling System Problem

On my wife's Van, I used as many different radiatior cleaners as I could find with different active ingredients (4 or 5), One at a time, I used each as they suggested. It cleaned it pretty good, the sludge was mostly gone.

Your thermostat should be changed too, try a Mr Gasket Stat, they are real nice.

It sounds to me like you've got an air pocket in the engine somewhere that's not letting coolant to flow into the heater core until it expands. My old 90, and my 95 have both experienced this problem. The red sludge sounds like it could be rust, so I'd go ahead and flush it again like racerraj suggested, but then try to get it to accept as much coolant as possible. Sometimes it seems like it won't take another drop, but you just gotta play with the throttle, and keep squeezing the upper radiator hose until the heat will blow hot, and it definitely won't take anymore coolant. It's a pain, but I'd hate to see ya end up with a cracked head from a hot spot not getting cooled. Who would eat all the rice by your house, if your 'Stang was down?