My Raider did not have the twitchy syndrome, but it did have some significant voltage drops. 28 years is a long time and connections exposed to heat, vibration and moisture can degrade.

Testing:

To load up the electrical I turned on the headlights, driving lights, directionals, rear defroster and any other loads that would be on when the ignition switch is in the run position.

To keep from killing the battery while testing I hooked up my charger. That way I didn't need to run the engine ( I was in a closed shop!!).

Then I took a volt meter and checked the voltage from the engine block to the negative terminal of the battery. I saw a few tenths of a volt.

To find the bad connection, I then measured across each connection ( from the wire to the lug and then from the lug to where it was fastened to).

Basicly, if you can measure a voltage drop directly across a connection, its bad. Especially on the main battery cables that are designed to handle starting current that are only running accessories during the test.

The connector to the body had about a .08 volt drop. At the engine I found a .05 volt drop.

I did the same thing on the positive side cleaning and or replacing or repairing any connections that had a voltage drop.

I made the same measurements under the dash.

There will always be some drops on long runs of wire that have high ampacities. What you want to find is resistance at terminals and switch contacts. I verified that my ignition switch was not a problem.

The end result was the dash board voltmeter now reads about 1.5 volts higher and my directionals now blink a lot faster.

The main culprits were the chassis ground and a connector under the steering column.

Things to note: The headlights don't run through the ignition switch so you don't need them on to test it ( ignition switch).

Head lights and driving lights are good to have on when testing the main battery cables. Unplugging and replugging a spade terminals under the dash may be enough to re-establish a good connection.

Check the connections and the condition of your fusible links.