Help your tech understand what the proper procedures are may help him figure out what is going on. At 15 years the ground ends vibrate loose and cause issues that float around. There a host of ground ends , connecting, etc that should be cleaned and tightened to eliminate poor connections. The only way to ensure proper connectivity is through a volt drop test or a resistance test. All just take a basic understanding of electricity and time. At normal shop rates and processes it is very time consuming so most techs bail and throw parts at the problem instead.
A few have posted that one area to look at is the Positive and Negative battery cables. One point is the jump point on the positive cable under the hood. They post that the end comes loose and causes a herd of issues. Others have mentioned the issues are found in the spare tire area as the connectors are discovered loose and not allowing a clean clear grounding of the main harnesses.

Vehicles that are exposed to high humid conditions have experienced multiple wire harness to connector issues. Starter, Alternator connections, water in the trunk causing MAJOR issues with wiring and electronic components. Take a few mins and pull the spare out and see what is going on if anything.

Good luck

__________________

2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green