1998 Ford F150 with occasional Starting issue

Hi guys/gals! I'm currently having a starting problem with my 1998 Ford F150 4.2L. My truck will start perfectly fine at times, and then once in awhile I will turn the key and all I get is a CLICK. But then if I usually wait a few minutes, the truck will start. But I want to get this problem solved.

So far I:
1) Put a brand new battery in
2) Put a brand new starter relay in (firewall)
3) Cleaned the battery terminals and connectors, as well as all the connections going to the starter relay.
4) Tightened up the truck shifter, as I heard sometimes a loose shifter can cause problems with the truck starting due to the truck thinking it's in gear instead of in neutral or park during a truck start.

Two years ago I put in a brand new starter/starter solenoid, so I'm thinking that that starter and it's solenoid should be ok. But at this point, maybe I should look to that as being a possible problem as well, even though it's fairly new.

Also, if there was a good way I could test the neutral safety switch and ignition switch I will try that as well. The problem is its hard to test all these components when the problem doesn't always occur.

Also when I put the voltmeter across the ignition wire going to the starter relay all I got was like 3.5 volts. But I'm wondering if that is the correct voltage when the key is in the run position, but wasn't actually being cranked at the time?

Any help would be greatly appreciated.

Could be high resistance in the battery cables, which you can check by voltage dropping from the positive battery terminal to the starter.

Could also be the starter/solenoid assembly. You mentioned replacement 2 years ago but you can always get bad parts.