yup could be a vacuum leak, could be computer too, i know xj's have a problem with the computer in cold weather where the comp does not turn on the fuel pump, had this problem with mine, just went to pick n pull and swaped out the comp. could be a problem with the distrbuter or perhaps coil, i would neet to take a look at it.
it definately is an odd problem, what part of the city you in?