The observation that with a known good solenoid switch the starter works sometimes and sometimes does not, suggests that the starter brushes are worn out or gunked up. Remove the starter and take it to an auto electric shop for a rebuild - or swap it for a good one at a parts store:

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Frank

'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum