I would try to isolate the problem first. First, prove that it's not related to the wipers and/or the radio. With the motor running, connect a 12-volt bulb across the battery terminals. Did that make the motor die? Try a bigger bulb. Did the motor die? If either bulb made it die, then it is probably related to how much current is being drawn from the battery. Now, try attaching a battery charger to the battery with the motor running. Do the same test with the bulb. Did the symptoms change? If having the battery charger prevented the motor from dying, then the alternator/battery probably isn't providing enough current to keep the motor running. By having the extra 10 amps or so that the battery charger added, the bulb didn't bring the motor to a point of losing shutting down. Try the wiper/radio test with the battery charger connected. Another test to do is to fully charge your battery for a couple of days and see if your symptoms still occur. If the symptoms are independent of the state of the battery and alternator, then it might be wiring somewhere.
Not all car shops do scientific analysis when it comes to diagnosing these problems. Replacing a part to see if the problem goes away is what they typically do. However, some intermittent problems merely become less intermittent that sometimes, it appears that the problem went away.