2006 Shogun - odd electrical issue

[FLGirl]

Hoping the hive mind on here or anyone who's owned a Shogun may have some pointers! It's a 2006 LWB 3.2 diesel. It's on 83,000 miles and has been well looked after.

In summary:
- New starter motor was fitted
- Took the car out 2 days out later, was out for an hour or two then stopped to pop into the supermarket
- Car wouldn't start (we were about 15 mins in the shop) but wasn't the battery
- RAC called and diagnosed that the exciter wire connected to the starter motor was showing no voltage. He jumped the car by connecting directly to the starter and we got it back and left it by the garage. It was a Saturday so they were closed but I dropped the keys off Monday
- Garage had the car for several days and said they'd had no problems starting or running it all week, and the exciter wire was showing power no problem
- He said he cleaned up the contacts and some surface rust/dirt from the connectors, as he'd seen this happen before when a new motor was fitted. I collected the car.
- Following day it wouldn't start, and was clearly the same problem
- A couple of days later the garage collected it and yesterday, after having it for a week and half reported exactly the same thing - it started all week without issues, they couldn't find any wiring problems and were stumped! He is currently fitting a bypass wire so should this happen again I can at least jump the car and not be stranded

My thoughts at this point are if it happens again to call an auto electrician out and see if they can trace the fault and get it fixed, maybe it's something really simple they've seen before?

Would appreciate any thoughts or suggestions though, am I/the garage just missing something obvious that could remedy this?

[MustangGT]

Not had a Shogun, but, as a general observation, a poor earth connection can cause many gremlins. I would check your earth straps to the chassis. Good luck!