Pretty sure there's something wrong with my car, mechanic brother says otherwise

My car (2000s jaguar) has developed a few very annoying problems since i first got it, it was perfectly fine for about a year, but now it occasionally jumps when changing gear and says gearbox fault on the dash, the first time it happened i thought the engine misfired or the propshaft snapped, it was that bad, it typically only does it when cold or when the weathers cold.

In addition it's had some minor random faults in the past that don't reoccur, that i think is a bad grounding or something, it said there was a problem with the suspension once even though it was completely fine and never happened again.

The thing i'm worried about is the gearbox because it's meant to have an oil change ever 68,000 miles and it's gone over that by about 3k miles, the dealer never told me about this despite claiming he has experience with them and likes the model (typical) i know nothing about the wizardry of automatic gearboxes but to me it feels like something gets stuck while changing gear and it suddenly becomes unstuck resulting in a jump, it stops doing it when it warms up or in warm weather because that changes the viscosity of the old oil?

However my brother whose been a self proclaimed mechanic for about ten years now, and has driven the car, says that i don't need the gearbox oil changed, that there's nothing wrong with it and that it's likely just a bad battery or ground? i'm a bit worried because he mentioned using a chatbot, he says because it hasn't jumped in a good few days it's not a big problem (been jumping on and off for about a year now, but doesn't do it as much in the warmer months)

Should i trust him? or is he talking out of his tailpipe? my dad says getting random faults and stuff is normal for a jag, i'm not sure who to trust