great. i am going to go on a limb here. its your gauge. and I'm guessing that thee is some type of ground that connects/disconnects when it hits a certain G rating
well, check for leaks in EVERYTHING; gas brakes oil tranny trans differential. and rust are major. check all the u-joints and try to drive it to see how it is because you may find something wrong with it
I'm going to say head gasket or blow-by. do a compression test and then do a leakdown test when the motor is cold and when it is warm. this way you will know if it is the head gasket or valves
its your t stat. there is only one solution if you motor won't heat up and the heat dosent work. your t stat is old and tired. they are cheap and easy to replace. i would not suggest getting one that will open wide at a tempature lower than "stock".and where from 190-210 is reasonable
what i did was i filled up a 5 gallon gas can and tied it in my bed and then when i ran out of gas i put the 5 gallons in, drove home and dropped the tank with my buddy ricky and problem solved.
i would clean up ALL filters, do exhaust, tb spacer can't hurt it in theory, but a good cleaning and pouring water down the tb to clean up the valves isnt a bad idea, as long as you don't put in too much
i do not see how air entering and vapros leaving will effect gas milage. seeing as on a full tank of gas you get about the same as empty. and the sending unit does not suck air up.