1999 Pontiac Firebrid Fluctuating Fuel Gauge

I have a 1999 Pontiac Firebrid, t-top, V6... standard everything. Twice now i have noticed that while i was in park, the gas gauge is going nuts. The first time it maybe wavered up and down within a quarter of a tank. Today i noticed it and it was going all the way from empty to full. I could hear the engine surging a bit, but not really. Not even noticable enough to feel it. I watched it thinking it would stop but it kept going all the way up to the peg after Full! It freaked me out so i turned the car off. I was just waiting for gas to come spewing out of the tank. Once i started driving it quit. It only does it while im stopped and in park. Im not getting any warning lights or anything out of the ordinary.

What could this be? Should i be concerned?

Thanks in advance!

Beck almost all GM fuel gage problems are the fuel level sensor.located in the fuel tank and may be part of the fuel pump assy