crazy problem.

Ok this is driving me nuts. My wife's car a 1997 Mazda protege with approx 60000 miles on it has a weird problem. I will try going through a step by step if it can help the diagnostics. First I changed the sparkplugs, about 1 month later I changed the rotor & distributor cap. Then it started... The car runs great, starts great, but after a while of driving maybe

15 minutes or so, it misfires. It seems like the whole car shuts down, the check engine and battery lights come on and the rpm goes all the way down. All this happens for a fraction of a second. The check engine or battery light do not stay on, and the car continues running. It can occur frequently like one after the other for minutes and then stop, and then car runs great. If it does while on idle after a few times of chocking the car will stall, but will re-start with no problems. The thing that gets me is that it can do for a week, day after day then go for a month without any hicups. Lately she can take the car drive it for a while, get to her destination and when she comes back to her car it will do it after a few minutes. It seems it will do after the car has been driven stopped and re-started again,before it had a chance to completely cool down, but it will sometimes do it during the initial drive. Being an intermitent problem Mazda told me that even if I bring it in for servicing they can't help me if it runs well while there. They said to wait till it always does the problem, but my wife is unsecure of taking the car for long rides. It seems as if the key is being turned off and on again cutting off all power, for a split second, and the dash lights come on like when you put it the key in start position(check engine & battery light). What can I do? can it be a bad distributor or rotor cap, but work fine most of the time? Some people told me a loose ground connection, but wouldn't this always make it happen. I checked the negative terminal and even if I remove it the car wil still run.... Enough said for now, if anybody has a clue, please let me know... Thanks