Overheating or Temperature sensor issues?

I own a 2001 Buick Lesabre. I spent a year abroad and came back in August. My car sat at a mechanic's for most of that time. When I got back, the problems were addressed with a new radiator in September. However, soon after, the red light binged at me, saying it was over-heating. I ended up getting new upper intake gaskets since there was enough apparent evidence that the problem was there, and not the gasket head, like the mechanic originally thought.
I got the car back three weeks ago, but I can't go more than 5-10 miles without the gauge going up to the red with the binging sound. If I turn the heater on, after a minute or two, the gauge goes back down, but I'm stuck driving with the heater on (which I won't want in the summer). During lunch, I popped the hood up and checked under there. The coolant reservoir was bubbling away and the fans were working hard. I stuck my hand between the fans and engine and could feel the difference between the two.
The mechanic had said something about the coolant not getting to the sensor, and I'm thinking that's possibly my problem. I don't want to go back to him, unless I have to, because he is not the most timely of people.
Any help would be great!
Thanks!

Well, for one thing, you don't go back to him, unless it's to get your money back....

Air bubbling in the reservoir is not a good sign.....

"The mechanic had said something about the coolant not getting to the sensor, and I'm thinking that's possibly my problem. "

Oh, my , god!

Who is this guy?

Very rare that a head gasket goes on these vehicles......were you you losing coolant prior to all these problems......

Was the thermostat ever replaced? Hard to tell if this "mechanic" ever checked the upper plenum for a leak, while it was off....