66 Bus fuel gauge goes to zero w/engine running comes up w/rpm

[DrDarby]

I've got an issue that's driving me nuts. I have a '66 Bus here that was fully restored elsewhere. Haven't seen any shortcuts or butchery, a very nice clean restoration with a new wire harness.
When you turn the ignition to the "On" position the gas gauge comes up to about a little about 1/2 tank. When you start the bus the gauge drops to zero but if you rev the bus the gauge comes up to about the same place little above 1/2 tank until the engine rpms drop. It had a replacement 12v gas gauge in the dash that internally showed a possible short so we installed a new Wolfsburg West gas gauge in it with the same issue. If we disconnect the gauge from the van and apply 12v and ground appropriately then ground the sender terminal it goes full. We've added a separate ground wire as it has a ground terminal, disconnected the + terminal and run power directly from the battery AND run a jumper from the gauge sender terminal to the sending unit all with the same results. Again, engine not running, key "on" gauge works, start the engine and gauge doesn't work unless engine rev'd. No big change in input voltage engine off or running.
Any thought?? Thanks
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Midwest Autosavers, Inc. Crystal Lake, IL

[Eric&Barb]

Have you cleaned the grounds at the sender?
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In Stereo, Where Available!

[DrDarby]

The grounds etc are all new but yes I did anyway.
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Midwest Autosavers, Inc. Crystal Lake, IL