2000 blazer ls

my gas gauge in my car bounces from full to empty all the time unless i fill it but after a few miles it starts again. any ideas?

The fuel level sender is faulty. It is attached to the fuel pump module inside the fuel tank and can be serviced separately with an AC Delco unit available at a GM dealer. If you replaced the fuel pump with an aftermarket fuel pump module, (Airtex, Bosch, Master, Carter etc) it's a common problem. The buffer that they use is, well, uh, cheap.

thanks alot i hate carrying a gas can with me everywhere i go