My temp gauge keeps teeter totting on the highway I can not use the ac at all now what would cause this to go real high then back down over and over?

I wouldn't drive it until you get it fixed. Whether it's a thermostat, water pump, head gasket, leak, or other issue.. just don't drive it and warp the heads (if they aren't already?).. I'm guessing head gasket since the load from the AC is making it worse? Try to get some "ball park" estimates from local places and get a feel for who has the most realistic prices, explanations, best bang for buck. maybe even google maps search (and skim reviews) for mechanics. HTH

Also read this: https://allwheeldriveauto.com/subaru-head-gaskets- problems-explained-part-ii/

Search the forum, as I've addressed this in length several times. Quickly: OE t-stats and water pumps are always fine, so ignore unless someone put in an aftermarket one on you; check rad for exterior front surface debris (leaves, mud, dirt); back-flush heater hoses (under windshield). Be sure to purge air fully. Still overheat under load? Sniff expansion tank for HC, indicating exhaust gas percolation. If so, you need HGs ($1600), suggestiing tossing this baker's dozen years Subie. But do the simple stuff first (but it ain't a t-stat nor water pump).

what is HC? Hydrocarbons?

Yup. First gases will bubble up under light throttle; eventually full- time at hot idle, with even violent explosions when revved up. One of the few times I wear eye protection.

also your gauge might be going out

In my 30yrs exp servicing thousands of Subarus I've NEVER seen a faulty temp gauge.

My 91 Subaru Legacy wagon temp started rising two days ago and it rises close to the H on the dash and then it will drop back down to medium again. Usually that's always a sign of thermostat. USUALLY* lol well I get a thermostat and I jack my subi up go under the car and see my thermostat housing... but it has three bolts and two different hoses, it's like a u shaped metal piping connected to the therm housing, and about 6 inches down its welded to what looks like my muffler or exhaust pipe. I'm so confused. I took the three bolts out and of course couldn't move the housing cover because it's welded to muffler/exhaust pipe! Well I wedged a brick in between the pipe/hose and frame just so I could try to get the old thermostat out and in the two wholes, no thermostats??? I'm so lost any one have any suggestions???

http://www.fixya.com/cars/t4878193- 1991_legacy_i_cannot_find_the_thermostat

Are you actually trying to find a t-stat in an exhaust manifold?

If he hadn't linked a pic, nobody would know.................Tim, I hope you find it and fix it and don't have any more overheating issues. good luck

Try taking a look under the LEFT side of the motor, where you'll find the lower rad hose emanating from the cast alum t-stat cover. You can remove it and test stove-top, but again I expect you'll find the overheating to be due to a blocked heater hose, radiator, or blown internally head gasket. But maybe you'll get lucky....

I did head gasket test, it came out good. My coolant isn't leaking and my coolant nor does my oil look milky so I don't think it's head gasket. It just started doing this two days ago. I haven't drove it cuz I don't want to worp anything or blow a gasket or screw my water pump up. I think it's either radiator hose, t-stat, or maybe air in my coolant flow?? I've replaced t-stat on a 5.9 dodge 1500 and water pump as well as a chevy cav. I've never had to go under my car to get to a t-stat housing. This is irritating. But thanks guys for the feedback! Much appreciated! I'm in S.C. Hopefully I can get the t-stat replaced today before the hurricane starts hitting us. Hope all you guys are safe if any of you are close to the southern east coast. Thanks again. I'll keep y'all updated