Erratic temperature change

[MIKEMAN]

My 95 Cutlass 3.4DOHC recently began to blow cold then hot air inside the car with the heat on maximum, on a 2-3 min cycles. Upon looking at the tempreture gauge it would also fluxuate from hot to cold on the same cycle.When the gauge read hot or above 100*C it would blow cold air, then as soon as the gauge dropped very quickly to cool, it would blow hot air.
Coolent was 100%
Replaced the thermostat day one; No change?
Replaced the Waterpump two days later; No change?
Going to buy another thermostat, maybe the one I bought was a DUD?
Any suggestions ??? I'm stumped ?

[OldSkoolGP]

It sounds like you have air in the coolant system from one fo two points: It's still in there from so work you did, or you have what I did, blown head gaskets. Hope that it isn't the latter.

[MIKEMAN]

I dont seem to have any coolant loss.
I removed the thermostat today and locked it in the open position (fail safe).
Refilled the coolant and attempted to bleed the system, drove the car several times noticed that there was no coolant flow or surge in the upper rad hose when squeezed.
The car runs cool on the temp. gauge, and does blow warm air(not hot) without fluxuation when the car is warm.
Could this be an air lock ?
If not, what are the symptoms of a blown head gasket ? What does it cost for repair ?

95 CUTLASS 3.4L DOHC