Speedometer Jumping Around....Why?

[69sixpackbee]

Heres the scoop. In my '72 I have the speedo jumping around and I am trying to isolate the problem. It is not the speedo itself or the cable as I have swapped them both. The speedo reads well while the truck is under way and is using power to do so but when I am at, lets say, cruising speeds, the speedo starts to drift downward consideralby but as soon as I tap the gas pedal it straightens out and reads correctly. This condition only occurs during very light throttle cruising or deceleration. I have a 4-speed in it and it has one of those offset little "boxes" that screws onto the speedo pinion on the trans and the cable attaches to that. I am assuming it is a ratio adapter. This has been on the truck forever. I hav a Dana 60rear w/ 3.54:1 gears and 16" rims so maybe this is what it is for but I am not sure it is the problem. Could this be the culprit or is it possibly the pinion gear itself?
The cable is well lubricated and no kinks whatsoever.

If it was just a speed indicating thing I could care less but since I am putting EFI back into it and will be utilizing a VSS module that attaches directly to the cable assy. I sure don't need this malady to affect my EFI function.

Any suggestions??
Thanx,
Bud