idle problem and lost of power with the a/c on

[danpaz_uy]

months ago my idle speed was too high, mechanic fix it, i think, not properly, now in cold weather, when i start the engiine, the idlle speed is the same than when the engine is cold, so somethiing is wrong, besides of this, you go from park to drive, the idle should go up, but it does not, ehen you turn the a/c on, the idle goes too low, idling goes to 500, i did recently a tune up, with the same mechanic, bas idea and last time too, and i have a hesitation, i think that is owed to the low idle speed, less than 700, and i think it should be 800 at working temperature, so please give me your suggestions.
is it any sensor for the transmission, Where is located, how can i test it?
what about the engine temperature, related with the idle speed in cold temperatures?
i think that the mechanic unplugged any sensor to fix the high rpm problem, and now im having problems with idle again.
by the way, no ses codes
the iac is new
new spark plugs, new wires, pcv valve cap and rotor, and all the filters, please help i love my gmc jimmy 95 2 dors 4*4, but im getting tired of the problems and i dont have a lot of money to spend, i like mechanics, and i have some tools and the haynes manual, so please direct me what to do , the more specifically you can, thanks in advance, Daniel