Join Date: Mar 2002
Location: Austin, TX, USA
Probably one of two things. Either a bad connection or a flaky voltage regulator. I presume the fuse block and harness are original.
Bad connections often exist at the fuse block itself. The fuse clips are probably a bit rusty . If you want to confirm this, get a digital volt meter and measure the voltage drop between the fuse cap and it's associated terminal. If you see anything more than a few hundredths of a volt you've got a problem. The fuse might also get warm on the end with the bad connection. If so, try a brass gun barrel brush to buff up the interior surface. I always like to follow up about any connection with dielectric grease. It helps keep oxygen and moisture away from the bright metal.
A couple of other places to check is the big terminal block behind the battery where the primary system voltages are connected. The buss bar on the horn relay is another primary point of distribution. But there are 100s of other possibilities. In all cases buff it, dope it and see if it improves.
If that all seems Ok, use the same digital volt meter and wire it up to your electrical system. See how much and at what RPMs your voltages are fluctuating. Depending on where you connect the meter you may find it is isolated to one circuit or common to all. The voltage regulator would become prime suspect in the later case.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI