it could be your altonartor
to do it yourself you need the folowiing, a decent multi meter with an accurate amps gauge or an amps gauge. the guage should be able to read up to 20 amps with a .1 amp...
Without being able to do a "hands-on" troubleshooting of your electrical system, NO ONE can identify the specific defect causing the problem.
There are probably many things which could cause this...