BMW Repair: thermostat bleeder screw, aluminum aluminum, screw out

Question
Car is a 1987 BMW 325I convertible. I'm having problems with it overheating. Was going to switch the thermostat but found I should really check for air in the system first by using the bleeder screw on top of the thermostat. But I can't budge it ! have tried every type of wrench in the box plus WD-40.  I'm assuming anything that gets hot & cold like this might just be nearly welded in there. Even if I change the thermostat, I'm going to need to use the bleeder screw.  Don't see any leaks but its running one notch below the red zone. I love this car and it has a great engine, don't want to blow it ! Every now and then the needle will literally 'jump' from that notch down to the half way point, then back up again. Does that indicate any particular problem ? thank you so much for any assistance you can provide.
Diane

Answer
You need to check to see if your engine is really running hot or if it's a gauge problem.  Your car had a recall in the late 80's  to have the gauge cluster taken out and 2 screws (one behind the gas gauge and one behind the temp gauge) tightened.  I can't think of any reason the screw won't come out, it's screwed into a piece of aluminum, aluminum does not adhere to steel very well.  

Does your electric fan in front of the radiator ever turn on? if so, because it's on A different circuit than the temp gauge you are in fact overheating, however if it doesn't, then it's either broken or your temp gauge is.  
Try whacking the top of the dashboard with your hand; does the gauge jump back down?

If you can't get the bleeder screw out now you will sometime in the future and therefore you may be best off getting a new thermostat housing.

Other possibilities include the fan clutch, restriction in the radiator or cooling passages, thermostat, water pump or even bad coolant.

To get that bolt off, try and find either a bigger wrench, a  weightlifter or get a pipe that fits over the wrench for more leverage, i believe aluminum becomes distorted when more than 45 ft lbs are applied, of course things are different when corrosion sets in.

But for sure check your engine temp with a thermometer to be sure it's not actually as hot as the gauge says, if you choose to drive the car at extreme temperatures, the life of your engine will go from 300,000+ to about 50-100,000.  the aluminum head expands and contracts at a different rate than the rest of your engine so it doesn't take long for a head gasket to get blown then the compression goes down on a few cylinders and the unbalance in combustion chamber pressures literally tears apart the pistons rods crank and bearings leaving your car steaming on the side of the interstate.
E-mail me if you have any questions.
-cheers

scheidmiwa@hotmail.com