Iroc not starting

I've got a 90 camaro 5.7 Iroc. I have been having problems with getting it started lately. As soon as i turn it over all power is gone. I have to go and wiggle the battery connections for it to get power back. When I go to start it again it does the same thing over and over. I have cleaned both battery connections so I know I am getting a good connection. The auto store told me the battery is good. Any ideas

Bad battery cables.

Didn't think about that one, thanks. I will take a look at them.

sometimes the battery can check out god but still have a bad cell and not have the umph needed to start the car. If teh battery is four or five years old its probably pretty much ahd it. Be sure all connections are good and tight. Also I have seen the battery cable appear to be good at the end but be completely corroded and inch or so away from the battery terminal so check those suckers out real god.