One thing you can do that won't cost you an arm/leg is to 1) remove engine shroud sheet metal and clean it up good, particularly the finned areas. 2) disconnect the negative battery term first and then disassemble all starting circuit connections - both hot and ground sides. Thoroughly clean to bare metal with emory/sandopaper all connection points, including both battery posts/clamps and chassis ground connection from batt and starter. Re-assemble, negative batt post last.

those 2 things may not cure all your current probs, but simply can't hurt - particularly on an older machine.

beyond above, starter may be getting tired or coil may be getting weak after it heats up....