Could be solinoid or brushes , or bad ground..
If you look over at the brake booster, right at the brake fluid tank cap level, just to the left on a wall, you will see a plug connector screwed to that wall with 3 wires .. this is plug X/27 and the V/WT wire is the feed wire to the starter solinoid.
If you hook a 12v test lamp to that wire [ back probe it], and that lamp lights every time you turn the key, you know all the electronics for starter are in tack and operational, but the starter itself, or solinoid, are at fault..
There is a NSS relay in the dash, but if this lamp lights , we know that is OK [ and you are lucky on that]
One last simple test before pulling the starter.
Next time it does it and you know you have power to X/27, grab some battery jumper cables and hook one end to the bat - [ neg] and the other end to the engine somewhere good [ metal] and retry.. this assures a good ground from bat to eng/starter..prob not the problem, but so easy to do, that it's a worth while try ..I like to be sure of the simple possibles....cuz I'm lazy
and don't want to get under there til I know I have to.................................................................................................. .................................................................