Make sure all you connections are clean (making good contact), the Fat Cat I just finished was doing the same thing until I literally scraped off the top of the battery terminals.
Jason
85 ATC 250R - restored stock other than 18" rears, nerf bars, Honda key switch, 14T front sprocket, and white tank and plastics (except rad shrouds)
87 ATC 200X - restored stock other than 18" rears and nerf bars
84 TRX 200 - rough but complete budget restore for wife and eventually daughters to bum around on
Eton Viper 50cc - oldest daughter's current ride