Hi Richard,
I had once similar symptoms, except my speedo was not acting up as yours does. I got Volt gauge things and dash lights dimming things going on last year.
It`s interesting because I experienced something similar to your issue, but only later I knew how it disappeared.
Ok so I had those two lower PS pump bushings VERY worn, cracked, so much deformed (still not visible at insp!) that ps belt was not running in a straight line (misaligned) ; all this made the pump tilt a bit, pulled upper bushing out towards front, a few millimeters, etc etc , leak some, etc, chain reaction
When I finally replaced bushings there, I removed both ps belt&alternator belts for service, changed ps bushings, fixed the leak, flushed atf fluid in pump, put belts back on, retightened&adjusted them both, positioned alt&ps pump...it helped belt align MUCH better, also my Voltmeter wasn`t going crazy afterwards (sometimes it was showing relatively too low, too high, too `SLOW` etc), instrument cluster lights stopped dimming on their own.
It suggests that in my case IF ps belt is too tight or loose, that can/may affect (I think) alternator`s effectiveness. Strange. That time I did so many check ups and fixes that I cant recall whether it was before
or after, but my battery needed fluid as well. And terminal connection wasn`t tight enough. I like going through things, inspect and hear things but problem is we always find something to fix.
Joy of having an old brick and Swedish-German magic.
Just an example
: my fuel gauge works totally opposite as it should: shows empty when tank is FULL of diesel,
until having only half tank of fuel, ...at about 7G left in car it moves to `Center position`, later it starts climbing to show `Full` as smoke flies out my exhaust (=tank goes empty). Strange. Hey It`s always very accurate at half tank!