I have a 2001 olds alero 6cyl. Already replaced coolant reservoir, thermostat, water pump and intake manifiold gasket. Still overheats. Sometimes it is good for a couple weeks but before long it overheats. Reservoir and thermostat are from Autozone. Any ideas?

There is a bleeder valve in the front of the motor. I used a 7MM wrench and did bleed the air out. I'm thinking perhaps replacing the canister/reservoir and thermostat with dealer parts. A friend did a block test? blue liquid that turns yellow when there is a head gasket leak. It stayed blue. This problem is so frustrating....been dealing with it since last October. There is a bleeder valve in the front of the motor. I used a 7MM wrench and did bleed the air out. I'm thinking perhaps replacing the canister/reservoir and thermostat with dealer parts. A friend did a block test? blue liquid that turns yellow when there is a head gasket leak. It stayed blue. This problem is so frustrating....been dealing with it since last October.