starting-94 merc G M

2 times in the last 10 mo. car refuses to to start after driving.I dont know if the solonid engages or not,but starter doesnt turn.after 20 -30 minutes of moving shift lever,letting car roll in neutral & drive it starts and runs fine.dont know if any of that was effective or just waiting would have done same.
I dont want to get stuck again,what parts should I replace first.
Thanks for any help you can give me on this.

when its in park and u try to start it does it make a clicking noise ? id say try to have some one inside the car trying to start it as u find the starter and try hiting it with a hammer and if it starts then u got a bad silinoid so good luck i had the same prob would turn the key and nothing so i got under had my dad crank it and when i gave the starter a tap she fired up i also have a 92 with the 4.6 so i dont no if theres any differnce but id try that and it does get old after like 3 times in the rain l

when its in park and u try to start it does it make a clicking noise ? id say try to have some one inside the car trying to start it as u find the starter and try hiting it with a hammer and if it starts then u got a bad silinoid so good luck i had the same prob would turn the key and nothing so i got under had my dad crank it and when i gave the starter a tap she fired up i also have a 92 with the 4.6 so i dont no if theres any differnce but id try that and it does get old after like 3 times in the rain l




Thanks for the advice,both times this happened I was alone and couldnt do much testing.I couldnt hear the solenoid click from inside the car or see what the headlights did.The dome light was not affected when trying to start.Today I cleaned the electrical connections on the starter.I guess ill have to wait to see if it happens again.
hanks again