PO122, 123 & 421 Codes. Engine runs terrible & Trany constantly shifts weirdly

Vehicle in question is a 2003 Galant ES 4-Cylinder, bought it at 220k Miles now has 270k Miles

1 Year Ago I had a PO123 Code, knowing little about Throttle Position Sensor (TPS) Troubles, I simply installed a new $15(Bought online) TPS and that solved the problem, for about a year.
A few days ago I started having the same weirdly shifting transmission & poor gas mileage troubles as I did 1 year ago, and along came PO123 again, this time then one-day later 123 code disappeared and along came PO122. So I figured the TPS went bad again.

This time though after we installed the new TPS everything acted fine for only about 2-days but the engine was Idling at 1,000.

It now being 5-days ago since TPS install the Car is acting terribly. The Engine runs rough and nearly stalls at Idle, Too High RPM at Idle (when its not stalling), Transmission shift flares at slow speed, drops in and out of overdrive at high speeds, poor gas mileage.

To be fair we tested the TPS 5-days ago and it tested fine using a Volt-Meter. We put a new one in anyway simply cause it solved the problem one year ago, this time it didn't solve the problem. We also installed the new one using a Volt-Meter (during adjustment it wouldn't go below .75 at one end or above 4.5 at the other, so we installed at a .9 to 4.3 position)

I've done some Web Searching on TPS troubles and it looks like it could be almost anything causing the problems.

Did any of you have Like Troubles that could steer me in the proper direction of what I need to do ?

Thank You

PS The PO421 Code I've had for over 1 year which was caused by a Friend that used an entire Can of SeaFoam to clean the Throttle Body, the 421 Code showed up the very next day.