I have a 2000 Chevy Venture van. It has the 3.4 liter engine. When I bought the car the guage worked fine. A while back the battery died and I had to replace it. After that the fuel guage hasn't worked properly since. It will read full (after I fill it up) almost all the time then, after about 100 miles, it will drop to empty instantly and then it will go back and forth from full to empty. I have checked the ground wire at the core support and it is making a good connection. This literally happened right after replacing the battery. Any help would be appreciated. Thanks.

It sounds like a defective fuel tank sending unit, and only way to check that is the removal of fuel pump