Seriously Need Help

First of all I want to thank everyone for all the responses. Have not found the issue yet. When I do I will post it so everyone else can be aware of the
resolution . No upgrades to electrical system except for a replacement several years back to a electronic voltage regulator. everything has been ok till now.
Just tried USA made Echklin (as original ) voltage regulator, with no avail. Zero starting problems so I am assuming the battery is up to snuff, but am going to try a load test anyway. For some reason I am suspecting a problem with the alternator. however I did swap it out with another one I had and it didn’t help. Its wierd because the meter reads a constant 3/4 charge at high RPM ( 40/50 +mph) I don’t want to fry anything , and below half way at a very slow idle.
thanks Mattax your info is helpful.