I also had this problem with my '77 auto.I fixed it about 10 or 11 months ago and it hasn't happened since.It's caused by heat soak from the exhaust manifold but is also wear related.
Briefly,you need to remove any scratches or grooves from the solenoid piston or plunger,especially radial grooves that indicate jamming.
Then,de-burr the lip of the solenoid cylinder or body.Give it a slight chamfer.
Fit your heatshield to your starter motor and notice the base of the solenoid is exposed to heat from the exhaust manifold.Extend your heatshied to fully protect the solenoid and the solenoid mounting base(where the actuating lever is)Use sheet metal and a few 6mm screws and nuts to attach it.Do not leave sharp edges.
Hope that helps.
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