transmission not engaging guys i need your help

[cyberfastpush]

1992 transport 3.8 156000 miles. my transport started to do something weird. it is randomly not engaging to any gear but if i rev up the rpm in a gear and change it to another gear it kicks in normal and drives normal. (no slipping even under a load) i test it by slowly putting the bumper to a tree and hitting the gas peels out no problem in both directions. this problem is happening very random no matter if the motor or outdoor temps are hot or cold. the fluid level is just a little over full when hot. i have noticed that when it is not engaging (in every gear) the the van does try to move but wont till i rev the motor up to 4000 rpms let of the gas and shift into another gear. then all the gears work perfect. like i said this is very random every time i tried to show someone it won't do it. (6 times) also im not getting any codes. (obe1) im good with mechanics and a machinist. as always thank you guys and if you have any ? about machining something im the guy with answers. i work with every type of metal and plastic for prototype machinery. thanks again

[BuzziBi]

On my Trans Sport there is a manual gear box so I have no experience with automatgear on this car. But! I've worked a lot on Cheverolets TH350 gear.
These gearbox was famous for its bad quality: o) so there was lots of fixes: o)
This is also a GM product so I could provide input on the error on the basis of what I know.
First: Have you changed the filter on the gearbox, if so, you would notice if there are impurities in the oil.
This is a result of turent filter and decreased oil pressure. Dirty oil is taboo for automatgear! Impurities in the oil leads to the risk that the valves in the gearbox will stuck, and will lead to uneven gear shift. Worst senario!
I expect that also this type of gearbox has the ability to connect a pressure gauges to check the line pressure, to confirm that you have the correct pressure on the gearbox. If this is correct, it is good everything else is just trifling
Expect that also this gearbox has a "governor" (a component that determines when the gear shifts is going to happen! this is a component that uses centrifugal force to control a valve in and out.) Stuck'd or slow "governor" can also cause uneven shift said it controls the valve that determines when the shift will happen.
The "governor" should be easy to access fore controll!
There are a well a of errors that can be manifested in a wrong gear shift, but one place must be the start! Good luck