1993 Engine Runs Rough One Second, Then Will Run Fine For Awhile, And Then Rough Again

[RamElectricalRet]

1993 D250, 5.2L, auto trans., 110K miles. I rarely drive it, it's garage stored and I only use ethanol free rec gas with fuel stabilizer. This truck is rust free and all the underside components (suspension, bushings, isolators, brakes, etc.) have been recently replaced - drives like a brand new truck...except....
Sometimes it runs great for a few miles then starts loosing power, coughs and spark knocks and sends small bangs thru the exhaust system. When its running rough it wont accelerate nor achieve higher RPM's. Downshifting and disabling the overdrive helps. Then all of a sudden it'll run great for awhile.
I've replaced the distributor cap, rotor, fuel filter, and O2 sensor. I haven't found anyone with an older DRB that can test the truck.
ANY IDEAS WHERE I GO FROM HERE? Replace the engine controller or some sensor?

 

[u2slow]

How about the ground jumper wire from the neg battery lead to the rad support?

Fixing that wire was like night and day for ironing out the stumbles and hard-starting i had on my '90 (360TBI).

 

[RamElectricalRet]

I'll look at it. Thanks.

Anyone else with ideas?

 

[RamElectricalRet]

Turned out to be a plenum gasket leak (also referred to as a belly pan gasket, connects the plate and the manifold together), and/or a bad crank position sensor.
The plenum gasket blew out in one spot, and while the intake was off the engine the crank position sensor could be accessed so it was replaced.
Installed the http://www.hughesengines.com/Index/products.php?partid=27091 and the new crank sensor and now it runs great. So - either one or both of these new parts fixed the engine missing/stalling issue.

 

[pacofortacos]

It was good of you to post the fix, so often people come here, ask questions and never post the fix that could help someone in the future. Thank you

 

[RamElectricalRet]

It was good of you to post the fix, so often people come here, ask questions and never post the fix that could help someone in the future. Thank you

You're welcome!

 

[Rlaman]

I had the same issue with my 93 W250. It sounded like it was possessed. It turned out to be the crank sensor. And it was hard to get to against the firewall.