2001 Pontiac Grand Prix

I have a 01 Grand Prix at 97,000 that I just bought a little over a year ago. I have had to have my radiator flushed twice in the last 4 months. The first time I did it was because they oil change place said someone put the wrong anti-freeze in it and there was sludge in the radiator (mixed green and orange) then the sludge returned even after the flush had been done and the proper orange coolant was put in. So we took it to a family friend who flushed it again and did a pressure check (because the low coolant light kept coming on, no leaks anywhere!) NOW my heat reader on my dash spikes until almost in the red zone and just before overheating in shoots immeadiately back down, also I believe some of the sludge has returned and there is a clicking noise going on. I really need to take it into a mechanic, but being a female and not accustomed to having to deal with the mechanics myself I don't want to get robbed into thousands of dollars of work (which I do not have). My boyfriend thinks I just need a new car, but I still own $6,500 on this car. Please help and let me know what you think is the problem with this car. Crossing my fingers it's not a head gasket!