Never heard of this before. Please describe what you mean by "shutting down" and how you determined that. How many recivers were you using? If you were using multiples, did they all act the same simultaneously? Were they AC or battery powered recievers? What lights were they on? Did they return to normal function when cooled? Did the flash fire normally from the test button on the back?

If the reciever's LED was blinking red in response to a CST, then it was not "asleep" or "shut down" (this assumes this is not a seperate symptom). If the recievers blink red, then there is a disconnect between the reciever's sync jack, and the light's trigger mechanism. This could be the the sync cord, or it could be an internal issue in the head or reciever.

When "on", the CSRB's blink green to indicate so. When they get triggered (by a transmitter or by its own test button) it will blink red in response.

Were these taken from a cold, dry location (air conditioning) to a hot, humid location rather rapidly? If so, you could try the "wrap-in-plastic-'till-it-warms-up" trick. Incidentally, camera bags are quite insulated, so if you keep them in your camera bag, they could still be fairly cool when removed, even after a couple of hours.

The idea is when warm humid air hits colder drier air, moisture will condense on/in the recievers (a glass of iced tea on a hot day does the same thing). If it is wrapped in plastic, the plastic acts like a vapor barier, allowing the units to warm up with less moisture to condense. You will want them to warm up to ambient temperatures before opening the plastic. They really don't get warm from being used.

This technique has been recommended for years for for cameras, however, I have never personally put it to the test.