1994 6.5 2500 4X4 Suburban Idle and Hunting for gears

[94blue]

This is a stock vehicle with 262,333 miles on it.
This truck quit going into overdrive. Sometimes it will and sometimes it won't. Sometimes while driving along in overdrive at about 50mph it will downshift out of the blue. Then a few seconds later shift back into overdrive.

Sometimes I can crank it up after a week or so and it will shift normally, But the same situation will also let it not shift into overdrive at all for the whole day??

I can even top off the batteries and it may or may not shift normally.

I have not checked codes (I'm rusty on that procedure), but any help would be great.

Thanks,
94Blue

[DmaxMaverick]

Start with a fuel filter change, check the lift pump operation and wastegate/boost control.