When I fill up my tank the gas gauge reads full, but as i slow down the tank goes to empty when I hit a full stop and will stay there for a good 5 minutes and make its way back up to full again...Sometimes tho, it stays on E and I never know how much gas I really have, what can cause this?

 

Sounds like one of two things. Either a worn out gauge (which isn't uncomon in these cars since the gauges suck anyway), or the float inside the tank is worn out and may need to be replaced.
It sounds to me like the float in the tank, but they are a pain in the nack to change since you have to drop the gas tank to get the sending unit (which is what the float is connected to) out of the tank.