why does my truck randomly shut off when i am driving? on 1992 Mazda B2600

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I have had my truck for a little over a month and a half and it has quit while i was driving it three times in the past 2 weeks. I had it towed into a mechanics shop the day after it happened the first time, and it ran fine, and wouldn't quit. They put it on the code reader, and nothing came up, but they did change the water pump. Then, it happened again twice yesterday, and i have no idea why. I know very little about vehicles, and i don't particularily want to take it to a shop again, cuz i don't have a whole lot of money to spend on it.
Any help would be greatly appreciated.
It is a 5-speed standard, if this helps at all.
(3) Answers
We cant really help, too many possibilities! Sorry. About all you can do is let your mechanic keep it, run it until it quits so he can check the trouble. Hard to find intermittent problems.
I have a 2000 Chevy Tahoe (new style, not the double doors). Anyway, my problem is that it randomly shuts off. I thought maybe I wasn't letting the engine warm up enough but I've left it on for a good 10 minutes before driving it and it still has shut off on me. I have about a 35-40 minute drive from home to work and just yesterday morning I was going up the ramp in the parking garage and the truck shut off on my, luckily no one was riding my bumper like people like to do when they are following other cars into the garage. When I got in my truck yesterday after work I did notice the lights would dim for a second then come back up to normal, this was while idling, and it did cut off on me twice in the process of getting out the parking garage. Now there are days that I dont have this problem at all so I'm not sure whats causing this.
check your ecm.