Car with unfixable TPS code

I don't know what to do, neither the dealer or a local shop that just fixes Subarus has been able to fix this car in more than 2 years.
2000 legacy wagon L I bought it new in 1999.
A few years ago, it started with TPS (throttle position sensor codes). The dealer replaced every sensor, the fuel filter, etc. Proclaiming it fixed each time. I couldn't afford to keep doing this, so I went to a local shop. They are much cheaper but still can't figure out what is wrong. Here is what it does: It is not the TPS or the computer. I'm 100% sure. I have replaced both with no effect. TPS has been adjusted for voltage over and over too, no effect.

When you first start the car, it often shows low voltage TPS code and most of the time the ATF yellow light flashes. You can't drive it with the ATF light flashing (it slams gears) so I have to restart. Sometimes more than 10 times to get the ATF light to stop flashing. It rarely shows this code at start up if the temperature is below freezing (32 degrees). Occasionally, but not very often. It does it most of the time if the temperature is above freezing.

It sometimes throws the ATF flashing light in stop and go traffic, always as a result of TPS code. It does it most at start up, less in stop and go traffic. It also surges at random at idle and in traffic. The only way I have found to get rid of the flashing yellow ATF code is to restart the car.

On the highway, it sometimes stumbles and surges when the throttle is off (down a hill) but rarely throws the flashing yellow ATF code.

So I have noticed, the ATF code (and of course TPS code) is related to the amount of throttle. And at start up, it is also related to temperature. When under load (giving the car throttle), it doesn't occur.

Local shop noticed some TPS voltage fluctuation and asked if the TPS was replaced. This is with a brand new (bought last week) TPS in place. So there is low TPS voltage at 0 throttle and minimal throttle, but not at higher throttle levels.

Voltage at idle off the alternator is excellent, that was a thought that the local shop had. But it is 13 to 14 volts even at idle with accessories running.

Last week it started showing the factory security system on (red light blinking in the instrument panel) any time the engine is off. Doors closed or open, unlocked. It's always blinking. It doesn't set the alarm off though unless the doors are locked like normal. It just indicates it is armed. The security light goes off when the car is started. I have no idea if this is related to the TPS code. I care much less about the security system problem then fixing the TPS problem.