engine idle, revving issues
I have a '99 2wd w/the 2.5 I4 engine, manual trans. The past few years, when the weather changes (Chicagoland), that the engine will rev up when I am shifting gears or shift to neutral when moving. It can go as high as 4000 but seems more common to go up to 3500, and will come back down after vehicle has stopped a few seconds. This year I noticed the idle was higher than usual-I have adjusted the screw on throttle body to try and lower but issues remained. The idle seems to change often (when sitting) following compressor clutch engaging and disengaging, which also happens very often. Also the throttle position sensor seems to go bad every 6 to 8 months. After trying to resolve issues at home I took it to my usual mechanic. I was told to replace the IAC and PCV because they were throwing off the fuel trim levels. So I replaced the TPS, IAC, and PCV with Ford Motorcraft parts; cleaned throttle body and replaced gasket; and replaced a couple vacuum hoses that were leaking or worn. This resulted is the truck running fine for a week or two before it reverted back to old symptoms. I have gotten check engine codes for TPS (a) low voltage and (b) intermittent and IAC circuit low. Recently I have also had trouble keeping my own reader connected, now this may because it emits a Wi-Fi signal to my phone, but it has become more difficult just to connect and yesterday I could not get it to work without disconnecting the battery and allowing everything, including codes, to reset. Upon having engine codes checked by local parts store, the person said that something may be wrong with the PCM because codes the codes were for new parts, the truck was running fine, and his reader said it could not finish its scan (unsure if this means anything). I don't believe I have any vacuum leaks, there is a very small coolant leak on a heater line, and no other mechanical issues. Plugs and wires were done over summer.
What am I missing, have I overlooked, can I check or replace at home? Can this whole thing be caused by the PCM? Given that I have adjusted throttle body screw how can I get it back to where it is supposed to be?
Please keep in mind: I am trying to keep costs down by doing as much myself possible, but I am novice to say the best with limited tools, experience, and expertise but due to an injury at work a couple months ago, I cannot afford a mechanic again.
Thank you for reading and replies,