95 Z71 SES codes not flashing

[AWESOME1]

My '95 GMC Sierra Z71 has been starting to have a few problems. It has 140,000 miles.
The Service Engine Soon light has been coming on and the truck wont shift and runs poorly. You can manually shift and still drive. Sometimes the SES light will go out and normal operation resumes. Occasionally the truck will stall, but start back up.
I checked the codes on two different occasions and it only showed code 12, nothing else.
If the SES light is on, I could shut the motor off, pull the ECM fuse under the hood, put it back in and the SES light would be out and truck would operate normally.
This has been going on, off and on for about 2 months. We only drive the truck about once a week.
Today the SES light came on and went back out. I checked codes when I got home. I got code 12, 13, 14, 15 and then the SES light just stayed on. I tried re-reading the codes a couple of times...but no more flashes, the SES light just stayed on. I pulled the test jumper and started the truck and it ran poorly and the SES light was on.
I checked codes again...only steady SES light, no flashes. I pulled the ECM fuse, same thing. I unhooked the battery for a couple minutes. Checked codes and only a steady SES light, no flashes.
I came inside for about an hour and tried to find info on the net. Found nothing, went back out.
I pulled the ECM fuse, reinstalled and tested codes. I now get only code 12 and truck runs with no SES light on....I only ran it for a minute.
Any ideas on what to check for???

Other problem I have posted is truck get's stuck in park with key on, have to start in neutral. Fuses good, brake lights work, I replaced brake switch about 2 years ago when installing trailer brake box.
Any ideas on this one? Can they be related?

Thanks for any ideas!