Hi: I have not been on since 2017, but I now have a problem - 1998 Jaguar XJ8 Vanden Plas

Here is what happened last summer.
1. I stopped the car at a red light; when light turned green I stepped on the gas, car did not move. I tried again, car moved, and the rest of the trip it worked fine.
2. I stopped at the stop sign; stepped on the gas, car moved forward a few feet, then stopped; stepped on gas again, car went forward, and worked okay the rest of the day.
3. I was parking the car on an upgrade, half way up the grade the car would not go any further. This time a message came on the dash "Gear Box Failure". Coasted the car backwards to a level spot; parked it; came back later and the car started and transmission worked fine. Left for home, pulled into the driveway (which has a small upgrade), transmission failed, coasted backwards and tried again going up the driveway and it transmission worked.

I did go to a local transmission shop, who said it might be some module, but would have to check. This has not been done since the car in storage for the winter, and will not be coming out until April.

Is there someone who would be able to tell me what is wrong. Thanks, Lorraine