Here's the big problem... it's not a constant issue, which makes snooping for info harder if not impossible.
since only part of the cluster stops working, i'm not inclined to suspect a problem on the PCI bus. this is what makes me wonder if you don't have a problematic cluster.
while i'm not wholly discounting the transmission issues, at this point I'd lay that at the feet of the battery/voltage stability issues... for now.

 

Reading your post thru a couple times, my guess is that it points to the PCM- since all other gauges & indicators work fine, even during the PRNDL3 lightup phase.

 

Let me clarify a few things: 1/26/24: I cleared the codes for the first time in years. Since then, I have not experienced a "limp home" only problem with the tranny. I have experienced having the PRNDL3 boxes all light up (twice), which ALSO results in the tranny getting stuck in " limp home" mode. These 2 PRNDL3+limp occurrences happened once with an older battery (1/26) and once with a new battery (on 1/28). Just wanted to clarify that the problem is not just with the instrument cluster- when the PRNDL3 lightup occurs and no speedo and no odomtr, the tranny ALSO goes into "limp home" mode. I can live without gauges if I have to, but the tranny going into limp mode is a deal breaker. Thanks for all the info and help!!

 

Happened (PRNDL3 light up) to me again today on 2nd start. Noticed that it lit up the gear indicators about 5 seconds after engine start. Will keep an eye on this to accurately describe symptoms in the future. Planning to drive around town as normal until the situation gets worse/ is more repeatable.

 

note whether or not the trans is shifting through all the gears properly when this is happening.
with my fancy scan tool I could probably diagnose the cause in 5 minutes once the fault happens, or at least have ruled out the transmission and/or ECU as the fault.
if it's shifting fine, I'm sticking with the "faulty cluster" diagnosis.
which cluster do you have anyways?

 

I suggest having the charging system tested. A failing diode pack can allow an intermittent ac (alternating current) ripple sufficient enough to cause a reset of some of the computer modules. Daughter's '06 Charger has a failing alternator with an ac ripple that would reset the instrument cluster (gauges drop to '0' and lamp check sequence) and would reset the emissions monitors to 'not ready'. Had an ac ripple on an '04 sebring that would spike the speedometer to 15 to 30 mph when stopped or in park.

 

Re: Sota's response: When the car is in the PRNDL3 - all 6 boxes lit up, I can shift to any gear- but it will be in "limp" mode (so car never goes into overdrive (tach is about 4000 RPM at 40 MPH speed (level ground)). Not sure what you mean by "which cluster type" that I have- what are my options? Curious about your "fancy scan tool"- would a transmission shop have a scanner that could diagnose this? Would be nice to know if it's the ECU or not.Do you think a faulty cluster issue would cause the tranny to go to limp mode? Re: ggoose response: would I see voltage fluctuations with ac ripple using a digital multimeter? I measured the voltage at the battery terminal to be 14.04 +/- 0.05 V. Agreed that something electrical is going on- the PRNDL3 lightup occurs about 6 seconds after engine start. i "think" I get 3? chimes when I start the engine, followed by a 4th one about 2 seconds later- right after the 4th chime is when the PRNDL3 gear indicator will light up, if it's going to happen. I guess it wouldn't hurt to check the charging system.

 

The voltage drop may not be enough to show other than a fluctuation on the meter but the control modules are more sensitive to the drop and duration of the ac ripple and will react quicker that any digital meter. Most meters are not instantaneous but average over a period of time.

A digital meter may show voltage fluctuations as the ac ripple would cause the voltage to drop but the ac ripple is brief and the meter may not show the ripples themselves that are typically measured in milliseconds and are not significant until they get into tenths of a second. A charging system test will look specifically for ac ripples as part of the test and record both the amplitude and length of the ac ripple.

If the transmission drops into limp-in mode there should be a stored diagnostic code. A stored code is typically not pulled by many scanners and many auto parts stores will not report a stored code unless requested. A stored code is one that was previously active but is no longer occurring. Stored codes are saved for 30-40 restarts unless the codes are cleared.