unknown malfunction

i currently have a 2002 Super VDP.......and last year i had a problem with it where out of know where i would lose power from the engine (while traveling on the highway).......and i would have to coast my way to the shoulder, and restart the engine. i took it to jaguar and they told me i had a messed up alternator......which is no problem

the same problem happend to me recently, and then the next week it happend again.......about 5 times in a row (within the same hour). the engine wasnt sounding too well.....and then youd feel a thud from under the car which gave u the indication that power is gone. i would get a "transmission error" and "failsafe mode" message. i think it was having trouble upshifting......so i had to drive it home at 40 the entire way....on the highway....which was terrible

today i took it to jaguar and they took it for a spin........found no errors. checked the computers........no errors. the manager told me he was gunna take it home with him to test drive it on more open highways where he can speed around a bit.......frankly i dont like that idea....but i had no choice.

has anyone experienced this kinda malfunction? or know whats wrong with my car?