Ignition timing mystery

I have a 1991 Toy V6 3.0 4x4 pickup. I still love it. It's old, new dents don't bother me, and with it's 35 inch tires it will go most anywhere, although not very fast. My son had it for awhile and so when I got it back I re-timed it. I jumpered it and set it up to 12 btc. Then I noticed that it maintains the same timing if you give it a little throttle until about 1400 rpm then the timing abruptly jumps to about 35 - 40 btc and holds steady there give or take 2 degrees as you go above 3000 rpm. It abruptly drops back down to 12 degrees again when you approach 1400 rpm. It does the same thing whether it is jumpered or not. it runs ok without missing but not great, not a lot of power. I can't figure it out what could cause this? Thanks for your thoughts.

 

Do you have any pinging or knocking? Any check engine light? Could possibly be the sensors in the distributor failing or the wiring. Possibly dead o2 telling the computer to stay in open loop.