L322 Seeking help - Gearbox Fault

Hi all, just joined and im seeking advice please.
I have a 2010 L322 3.6 V8 Auto
Xmas 2024 it had intermittent gearbox fault with restricted performance multiple times whilst driving. Stopped and restarted which cleared fault, did some research, lead me to the battery which wasn't quite enough on the voltage. New battery fitted 27th Dec 2025. Then a week ago Feb 2025 the same faults 4 times in one day all at around 50mph. Booked into garage, had gear box service carried out (it was due & hoped that would solve the issue) but under diagnostic test no code relating to fault code stored. 4 days after gear box service the gearbox fault reappears only the once and after driving for over 2hrs. Spoke to garage who are looking at it again Thursday this week to see if there is a code...if there isn't, whats next?
Thank you for your time.

What diagnostic are you using? There's must be a fault code if you're getting restricted performance

kermit_rr said:

What diagnostic are you using? There's must be a fault code if you're getting restricted performance

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TBH I have no idea, but can ask. I left the vehicle in their hands at that time, they said there were no codes stored relating to that fault and suspect that it will be the same on Thursday. I just don't want to throw money down the drain. They are LR/RR specialist hence using them, good reviews....
I've searched everything I can about this and nothing coming up other than what I've already done

Simple things like the earth strap from engine to chassis failing can cause problems. Is the alternator charging properly?