1999 alero changed thermstat and the car will intermitantly overheat there seems to be no pattern.

BLEED AIR OUT COOLANT SYSTEM.WAIT UNTIL ENGINE COOL DOWN.REMOVE COOLANT OVER FLOW PRESSURE CAP.IF YOU HAVE 3.1 OR 3.4 ENGINE OPEN AIR BLEED SCREW ON WATER PUMP BYPASS HOSE.OPEN BLEED SCREW.WATCH COOLANT IN OVER FLOW AS YOU OPEN BLEED SCREW COOLANT WILL DROP IN OVER FLOW JUG. AS IT DROPS CLOSE BLEED SCREW ADD MORE COOLANT IN OVER FLOW JUG DO THIS UNTIL COOLANT LEVEL STAY AT COLD FULL MARK.WHEN DONE TIGHTEN BLEED SCREW DONT OVER TIGHTEN IT OR IT WILL STRIP OUT BREAK.CRANK CAR UP UNTIL THERMOSTAT OPEN THAT WHEN TOP RADIATOR HOSE GET HOT.WATCH COOLANT GAUGE,IF ENGINE TEMPERATURE RISES TURN OFF ENGINE WAIT UNTIL COOL.REMOVE RADIATOR PRESSURE CAP USING A RAG TO KEEP FROM GETTING HANDS SCALED.ADD MORE COOLANT IF NEEDED WHEN ENGINE COOLANT STOP DROPPING IN OVER FLOW JUG AND ENGINE DONT OVER HEAT AIR IS OUT OF COOLANT SYSTEM.