Your not giving a lot of info,
Single tank or two tanks? Does it on both or just one? Did this occur with the old sending unit?
Regardless I think you need to test the gauge. Standard testing is unplug the sending unit wire, turn the key on. The gauge should read well past full to somewhere around the 3 o'clock position. Now ground it out, with it grounded out the gauge should hit E, or close to it. Run a wire directly from the negative on battery or clean up a spot of bare metal on the frame do NOT use the gas tank or the gas tanks ground wire. If the gauge works as it should look for a bad ground wire off the gas tank. If the the ground wire looks good clean up both ends of the wire, you need bare metal, good connectors, and good wire. Put a little dielectric grease on those connections so they keep working good for a long time to come. If none of those are your problem then you got a defective sending unit. Remember that even though new should mean good it doesn't always. We have had good luck with Spectra sending units at the shop. Good luck, it's a pretty simple system and I'm sure you'll have the issue isolated in no time.
I've never seen a fuse box or power supply issue causing what you describe, obviously that doesn't mean it isn't possible I'm just mentioning that. The sending unit is nothing more than a variable resistor and is suppling a varied ground to that (typically) pink wire on the sending unit.
