Fuel gauge question? 1985 Chevy C30

[motornut]

i just replaced the tank and sender in my 79
both Spectra brand (no bouncing issues before or after)
i had a bouncer in the 78 tho
Spectra brand again but
changed the cluster from idiot to tach/gauge
added some grounds behind it and the bounce went away

when i first looked into the bounce ,adding a ground was suggested here
not sure why it didn't help me when i test grounded it
and why it stopped when i added the diff cluster
i can only suggest there must be feed back disrupting the path to ground
seeing they all go to one wire
and it can't hurt to add one...
when I ordered this one my parts guy said he never heard of this issue and he sells a lot of the Spectra brand
and tank is not baffled