1994 Ford Probe stalls while driving

1994 Ford Probe with 84,000 km on it. My car stalls while driving - usually 10 or 15 minutes into the drive, about 70% of the time. When driving the same route, it frequently stalls at roughly the same distance, but not always. Doesn't matter what speed or I can be stopped at a light (however, I've been avoiding driving anywhere over 60 km). Stalls once, can sometimes restart while still in motion, but now have to pull over and restart. Was only happening once per trip, but in the last two days it stalls two or three times in a row within a minute of each stall. Sometimes it idles rough, too. My mechanic has done three diagnostics and there are no codes coming up and he is baffled. Replaced the battery as it was showing low voltage, but did not fix the problem. However, it did stop the occasional whining I was getting from the alternator belt when starting up, so I don't know if it's related to the stalling problem. Can you give me any clues what to look at?