Everything just died!

Any help from anyone would be greatly appreciated. I bought an 88 firebird w/ a 2.8, 5 speed to race and it was running great until it just died, no power at all not even dash lights. The next day the car ran great for about 5 min. then died again. The day after that it ran for about 3 min. then died. Now it won't start at all. It's not the battery, checked that. Anyone encounter this problem? I got a mechanic coming over to check it out but don't think he's going to figure it out. Please help!!! Race day is on the 28th of June.
Thanks,