Intermittent Cranking Issue

[sanibel sailor]

I have a cranking problem that is driving me nuts.

When it cranks, it spins merrily. starts right up and runs great. Seems to have an intermittent cranking issue. Other times, I get a resounding "clack" out of the solenoid and volts drop slightly, but no crank.

Just installed a completely rebuilt 4.108 in a boat that had set for a few years. Starter was rebuilt and a new solenoid put in. House battery bank (4 golf cart 6V's) is only a couple of months old, and initially had an older start battery, but started 5/24 after charging it. Started several times in a row. Overnight, it would not start in AM, even with good voltage showing. I recharged battery, rewired new hot lead to start switch from terminal block, still no go, so I put in new start battery. No luck. Batteries are fully charged, reading 13 plus volts.
This AM I cleaned all terminals of house and start batteries, cleaned all lugs at the battery switches, on the alternator hot wire and the battery isolator as well as the ground wires to the block. Fired right up, I am thinking problem solved, then later no cranking. So, I assume not enough volts to solenoid, I replaced starter switch, tried a wire direct from distrib panel to switch. No luck, only a click from solenoid.

I jumped a wire from the battery terminal on the solenoid to the switch terminal on it, just a loud click and a hot wire, but no cranking. Wire from terminal on battery selector switch same result.

Tomorrow I will take my little jump-start battery pack and try hooking that to it.

I am thinking that either my starter or solenoid is bad, in spite of being new or rebuilt. One item of significance is that a bad starter or solenoid is the responsibility of the engine guy, any other problem is on me. This started as a cheap and simple running take out swap and ballooned into a total rebuild. They have more hours in it than planned and I am over budget, so I am inclined to try everything before potentially running up my bill higher or asking them for more unpaid effort. Their shop is an hoiur away so a quick "drop by and check" is challenging. Yes, we are still on good terms and they are willing to check it again if I cannot figure it out.

Anything else to look at before I blame the starter or solenoid?