Race Report/oddball chain-drop issue

Here's the situation: Cyclocross season started up yesterday* (How do you say, "Whoo hooo!" in Belgian?) and so I, being the larkity fool that I am, built up my bike on Saturday night for racin'. The frame is a vertically dropped-out Lemond Poprad--yer basic CX frame. I (in my usual spirit of cussed bricolage) decided (rather a long time ago) that I'd build it up (and race it) fixed if at all possible. I tried out a pile of chainring and cog combinations with a length of chain and found that 41 x 17 was the sweet spot: no fiddlin' needed and perfect shain tension. I procured a new chain (and a needed half-link), and a 17t cog (I'd been testing gearing on multi-speed freewheel), and went to assemble the whole thing.

No dice; chain too short. Okay, not a hard problem; it's the difference between a new chain-and-cog and an old one; I'll just grind a flat onto the axle so that I get the 1 or 2mm of adjustability I need to get into the dropout. Piece of cake; I didn't even need to grind past the threads to get it into the dropout.

And all was well; I raced in the Masters 30+; with a bit of rain sprinkling the course and for just a taste of mud and wet leaves, and performed embarrassingly as expected (hey, I was just getting my bearings back; I'd not ridden fixed in a year or two, not raced in two years and, oh, yeah, not trained either). All was well, my focus was on the 4's (beginner) race in the afternoon.

By the start of the 4's, it was really raining. Ahh, a mudfest! I remember I used to do well in these! (And my chain felt a bit loose. But how loose could it be? It was brand new, cog was, too, and I knew that the whole thing was pretty tight to go together. Besides, I was in the staging, and I damn sure wasn't going to lose my starting position in the race I had a chance of finishing in the points in.)

But I started almost as tentatively as the Masters race; where was the aggression? Then, maybe 500m in, it all clicked again: Not winning? GO FASTER! Not passing someone? GO FASTER! Braking? STOP BRAKING! I was really feelin' it; the virtue of riding fixed in the mud---rather than front-braking in the corners, skidding the front wheel (at worst), understeering, and scrubbing a ton of speed, I started challenging myself to take all the turns with no handbrake; slow as needed with the rear wheel to feel the slip in the mud, to carry more speed, to slip for oversteer rather than understeer…. It was going great.

Something had to go wrong. And it did: Chain drop, coming out of corner (I'd been resistance-slowing into the corner). I knew it was a bad sign; I could crank the chain back onto the chainring like a derailleured (derailleurisé?) bike. It was bound to happen again. And while it was fun to pass the same fifteen people, drop the chain, and repeat (twice more), when my rear wheel started falling off (and falling off again), I knew it was time to DNF.

But a blast was had by me--anytime you have to come straight in the basement door, undress next to the washing machine, and rinse your legs in the utility sink before entering the house, you've had fun.

QUESTION.
How did my brand-new fixed drivetrain go from sweet-spot tight to chain-droppingly loose in the course of 10K on roads (on the way to the race) and maybe 10K of racing on dirt, grass, and mud?

My first theory was initial chain stretch, maybe wear-in (it's a KMC chain, IIRC); my second theory is that the cog (a black Eighth-inch brand cog) was powdercoated all over, and I've worn the powdercoat off the teeth. I've since readjusted the wheel back in the dropouts, maybe 1 mm back, and it seemed nicely snug again--but it reared its ugly head on Wednesday morning again.

New theory, suggested by an iBOB: unround chainring and unround cog combining to give loose spots. Maybe a slightly bent chainring, too, with a tooth coming up outside the sideplates all on its own.

Other new theory (also via iBOB) is that the powdercoat on the sides--the clamping surface--of the dropouts is letting the axle slip forward.

Last new theory (home-grown) is that the axle is slipping up as I go over the bouncy ground, and into the middle of the dropout, no matter where in the dropout I start it out.

(New datapoint is that the chain is pristinely new-length (12" per 24 links).

Thoughts?

*wrote this a-Monday; late posting to the blog since this is my last venue for readership.