over heating/thermostat not opening

[DemonGT]

the other day i went to a freinds house and when i got out i heard a bubbling type sound coming from the front of my car...i didnt think much..when i drove it again it started over heating..going over 200 and when its warm it usually dosnt go past 160 i have a 180 thermo..we added some water to my radiator and the thermo opend and it seemed fine..now it was runing at 180..like i said..usually dosnt go past 160..we let it sit there awhile and the thermo wouldnt open..why could this be? just a bad thermo or what?

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'91 GT,91K,T-5,air silencer removed,HD clutch,K&N air filter,Mac long tubes,Cat-back,off road H,FMS 9mm wires,180 thermostat,Competition gauges,C&L 76mm MAF,Accufab 70mm TB,TFS street heat intake

Waiting install:Nothing

Future Mods:3.73's,Pro 5.0,TFS heads,TFS stage 1 cam,lakewood struts/shocks,nitto DR's
ICQ#35670781