Not to get too argumentative about it because I really don't care all that much ... but the Canon flash most certainly does have a capacitor threshold shut down, and I've seen it happen with my shooting partner's 580EX, more than once. Both of us had our flash units shut down with-in minutes of each other at one hot summer day's wedding while shooting a back-lit scene ... his a Canon, and mine a Sony. The Nikon SB900s also used to shut down on me, and did so much more frequently because Nikon set the threshold to low. I believe you can by-pass it on the Nikon, or they did something to raise the shut-down level, can't remember exactly.Luckily the canon units don't have the auto shutdown of the sb-900 which has my mentor and friend tearing his hair out (he's a PJ). I'm using 35mm cameras so I need far less aperture and hence less light, my dual speed-lights in a brolly are more than enough which is useful.
I know you use an older Canon unit as your main flash, (which I never had shut down), and using two TTL speed-lights in the umbrella helps lower the need for full output. But, trust me, the Canons WILL go down if they get to hot, and it takes upwards of 15 minutes to get them cooled down and back on line.
FYI, I use the strobes at weddings with 35mm mostly, the MFD is more for portrait work ... although that may change now that the S2 is in the bag. However, I agree that dual speed-lights and umbrellas work just fine at weddings ... but as full output studio strobe replacements they don't. It is to taxing on such expensive tools.
Anyway, back to lighting of all forms ... I hope to post a few new "lighting shots" by this week-end after a pregnant client session ... with the girl's permission that is I forgot how much I love working in the studio. I've become a photo hermit this winter ... :ROTFL:
Best to you Ben,
-Marc