jmuva44 said:
This sounds like the most obvious reason, air lock.
The time you said it takes to start overheating is just enough where not enough coolant will start causing problems. Especially if the air is trapped at the bottom of the radiator return inlet. At this point, it would seem worthwhile to flush the coolant system and start over.
If you take this advice, after filling up the reservoir, leave the radiator cap off for a while, start the car and let it gurgle for 10 or 15 minutes. This will be enough to drive around the block a few times and see if the problem is still present. In doing so, the needle will bounce around with various temps. This will require of you to watch it for a couple of days and maintain the proper level of coolant until the air is relieved. Good luck.
PS. Make sure the coolant mixture is also right, eg. 50/50. I never really cared for the pre-mixed stuff.