Gas gage stays on full while car is running on 2000 Buick LeSabre

I keep my car on a full tank of gas because not know how much gas I have.I set my trip miles to 0 after each fill up and around 100 miles I refill tank,well I went to 145 miles and the gage droped down to a half a tank and started working not sure if it was the right amound of gas doe but it was working. but when I put more gas in the same thing after 145 miles it start working again.well now the other day I had to take a long trip and filled up and about 45 minutes later the gas gage droped down to emty while driving 70 mph and a few minutes later shot back to full and the same thing on my way bach home.could this be a bad sending unit or in my dash board?

Asked by for the 2000 Buick LeSabre
have a gauge sweep done on gauges to see if its a gauge or wiring issue b4 repl sending unit
2 more answers
probably a bad level sensor in the gas tank
could bee what goodguy said or a short in the wireing be carefull .
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