I don't know if my experience with an erratic cruise will apply to your situation...but who knows, and maybe it will help someone else.
I got into my speedo head to fix my slipping odometer (I found a fix if anyone is interested)...while relubing and adjusting everything in there, I inadvertently loosened the speedo damper so that every irregularity in the cable or gear in the tranny was displayed on the speedo.

So look to see if your speedo needle is rock solid or the amp will see erratic info. Could even be a speedo cable or tranny speedo gear problem.
Richard

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'83 300 TD..."Helga" Norse for holy... a liittle rust problem
'85 300 TD...A $200 rear-ended total..being transformed into a pick-up truck
'68 Ford Ranchero GT...a pick-up truck that Ford transformed from their wagon