Loose gas gauge

My 1990 Jeep Cherokee gas gauge level fluctuates about 1/4 tank. The level goes down when I accelerate, and up when I decelerate. I had the pump replaced a few weeks ago, along with the entire sending unit, and at first it worked fine. Only after filling my tank full did the problem begin. I took it back to the shop, and the mechanic told me it has nothing to do with the parts he replaced. He says it's probably an unrelated electrical issue dealing with the gauge itself. I'm wondering if he's jerking me around. Thanks for any help.

I do not think this is a problem, as some of the tanks are only 15 gallons.
All of my Jeeps do this, my 87 as you described. Turns and braking and acceleration makes the gas slosh in the tank. this causes the float in the tank to ride the waves going up and down like a boat at sea.