1990 Jaguar Sovereign Cutting off unexpectedly

Have had this car for 3 weeks and have had no problems up to this point. Drove car 350 miles and just when I was about to turn into my sister's house the car sputtered and died. Had it towed to a mechanic (Jaguar) he used his diagnostic machine and said it showed "computer problem". He said he was sorry that he could not do anything for me but to turn "service engine" light off. I came home from her house (another 350 miles) and 5 miles from my home it did the same thing again. I called AAA and while waiting for them, I tried to start it and she started up right away! I am scared to drive this car anymore, please help me, I love the car but am very disappointed! Thanks so much for your listening to my problem!