Very simple - YOU NEED AN ENGINE GROUND STRAP!
Otherwise, all those chassis grounds, ie. anything that grounds through the chassis; your stereo, the fuel sending unit, gauges, ignition, etc, may not function CORRECTLY. There will always be some minor engine/chassis grounds, but how does the ground reach the negative side on the battery unless it (the engine, or negative battery cable) has a chassis ground strap? Electricity operates on a circuit. If there isn't a good way to complete the circuit, things won't work right, or at all. A ground strap is an *excellent* idea, and they're only a couple of bucks from Pep Boys or Kragen.