Fusible link?

Be absolutely certain on those blue ones (actually any of them) that you scrape paint off the mounting holes and use star locks to insure the case is grounded

If you have that done, the rest is very likely harness voltage drop. You can check for that by temporarily rigging a test wire, come off something 'solid' like the start relay big stud, run a fused wire over and hook direct to the VR ignition connection. Be sure to unhook it when you shut off the engine.

If that test wire "fixes" the problem, then that added relay I told you about should settle things down