It started by scaring me off one of the local express ways when i looked down and saw my gas gauge in the bottom of the red. Now I was sure I hadn't forgotten to fill up that week, but you never know if you spring a fuel leak. So I took the next exit off the road and once off the road the gauge came back to where it should have been. It seems to happen more when I have less rather than more fuel in my tank. But It's a fairly regular problem, happening once a trip or so, (1 trip = 40 min drive to work)
I'm thinking it's the sending unit but I was wondering if there is any way to check this. I am fairly good with electrical/electronics stuff so if it gets down to re-wiring I can do that.
And if it is the sending unit, what can I look foreword to doing next time I get a few days off.

Apply your electronics skills first: substitute for the sender using a 68 ohm 1W resistor between the gray wire and ground. If your drive thereafter has a stable ~3/4 tank reading, you can look forward to removing the in-tank sender/transfer pump assembly. Hope you're lucky and it is only a loose nut on the back of the gauge where it mounts to the pc board in the cluster. If you post back, lots of info is available to help with the sender replacement; not a trivial job.
--
Art Benstein near Baltimore

The sending unit also includes the tank's top cap (proper name is unknown to me), complete with terminals and fittings for fuel and electrical in/out. If your tank's top cap looks awful and rusty - like mine did - you'll be glad to be replacing it. Eventually it's a source of leaks and/or electrical failure in the pump and level sender.

Access in the sedan is a bear. In the wagon it's not so bad. There's a special tool to remove the bung nut that holds down the cap, but there are ways without the tool.

Have you noticed if the temp guage also fluctuates at the same time? There is a step down resistor on the back of the dash in mine because both gauges run on 8 volts not 12. Mine went south and both gauges would jump at the same time.
However all float gauges fail in time and yours is 15 years old now.
I have had to fix our toilet more than once in the last 15 years.
The access is on top of the tank but is probably confounded by an in tank fuel pump on yours.