Since you did almost the whole dive and had no issue until you were down to 700 psi, some of these proposed explanations don't make sense, to me. But then, I'm barely less a newb than you, Ski.
The explanation about a piece of "gunk" getting there seems plausible. Especially after my local dive shop shared stories with me about servicing people's regs that had been vomited through. Even though they were washed out afterwards, there was still "pieces" in there that wouldn't go away until the reg was disassembled for a proper service. I don't mean to gross you out (too much
), but who knows what someone did with the regulator you were using the day before you used it?
The other explanation that comes to my mind is that some second stage regulators are unbalanced, some are mechanically balanced, and some are pneumatically balanced. First stages come in unbalanced and balanced as well, but since your octo worked fine, that implies that there was no issue with the first stage. Anyway, the behavior you described sounds like what could (?) happen if the second stage you were using was unbalanced. Unbalanced means that the performance will drop off as the pressure coming into it from the first stage gets lower - which, in turn, will drop off as the tank gets low. From what I know, it would be strange to have an unbalanced primary second stage and a balanced octo, but who knows what a dive operator will do with their rental equipment....
Anyway, very cool that you enjoyed it all and are still eager even after that (potentially scary) situation.
BTW, DRiS currently (well, still did as of late last week) has a sale on a really nice regulator set, the Hollis DC3 w/212 second stage, for $199. It's a balanced, diaphragm, fully sealed first stage (which means, good for cold water - i.e. below 50 degrees - and super low maintenance), with 2 high pressure ports (gives you the option for a pressure gauge attached by hose AND a wireless pressure gauge transmitter, if you ever want to do that). And a pneumatically balanced, fully adjustable second stage, also good for cold water. A hard deal to beat. So hard, I bought a set last month and did 3 days of diving in the Caribbean with it right before Christmas. Those were my first dives since I got certified, but the reg set worked great - definitely better than anything I used during training.
