2000 Oldsmobile Silhouette No Heat and Overheats

[Buckweat54]

I have a 2000 Silhouette that the heat went out on and it also started getting coolant in the oil. I figured it had a stuck thermostat and the head gasket was leaking. I went and bought a new Thermostat, Bars Leak Head Gasket repair, and some coolant. I removed the old thermostat and put the housing back on, then I completely flushed the engine, heater core, and radiator. I went through all the steps with the Bars Leak and could actually see it sealing all the leaks on the head gasket. Flushed the system again and put the new thermostat back in and filled the system with new coolant and proceeded to run the car and top off the coolant. Started the vehicle and at first I still had no heat, but about 20 minutes later it started working great. I drove the car for 2 days with no problems. On the third day the heat stopped working and the car started overheating. I can only drive it a couple of miles before it overheats. I haven't had a chance to get in and check the new thermostat, but is there anything else that could be causing this? And how do I fix it? Thanks in advance.

[LMP]

UNhappily, head gasket problem with the 3.1/3,4 is pervasive and probable, and cannot be dismissed, rather a WHEN than an IF. 3 years ago, I had told my brother to change his 2002 for another car before it happens...and 3 weeks later, it was too late. Miracle sealants may work sometimes, or for a while.
Open radiator cap , fill to the rim, and run the engine, look for bubbles. That would be a telltale sign. A CO2 test of what vents out from the radiator can be done also, there are kits available for that, kind of juice that changes color when contaminated by CO2.

Intake manifold gasket can leak also, but that has no influence on heater performance