So what I think we need more of on this board is data on timing and reliability for restaurants that people know very well.
I think this sort of stuff is less of an issue with high-end restaurants, but on the mid-and-lower end, quality can vary widely from cook to cook, and from one time of day for another. I think a lot of them can't afford to prep each dish at each order, so hitting them at high turnover times is important.
I think their margins are just so small they can't afford to do what it takes to be good all the time.
I've just been finding that some of my favorite restaurants in town are frustratingly elusive.
I'll give what I know on some Chowhound favorites:
Guelaguetza (8th street). I think their moles and other sauces get prepped in large batches, and on a slow night just sit there. Almost invariably, whenever the restaurant is crowded the moles taste much more sparkly and fresh, and when the restaurant is empty, towards the end of a slow night, the moles are tired and sad. Strangely, the most reliably empty time is Friday around 7 PM, and the most reliably packed time is a Sunday night. I've found this to be true for a lot of the Mexican neighborhood restaurants I frequent - empty on Friday nights, packed on Sunday nights. Amy, who has lived in Oaxaca for a while with a Oaxacan family, gave me some complicated explanation involving Catholicism and family values and Friday being the night when all the families hired babysitters and went out *dancing*. Anyway, I avoid it on Friday nights. Also, during the weekday, it's crowded early - I mean 5 or 6 PM, and dead (and stale-tasting) by 8. I have no lunch experience there.
Also, chorizo quality there varies unpredictably. I think they make new batches irregularly; sometimes the stuff in the restaurant tastes just like what happens to the chorizo I buy from there after it's been in my fridge for a week.
Juquila: Tricky. Judging by the variation there, there are two cooks, with different attitudes towards the sauce. One is incredible, the other mediocre. The mediocre one especially destroys the chiliquiles, which come out soggy and fatty, instead of the incredible one's crisp, just-soaked effect. The mediocre one works all lunches and occasional nights. The lunches there can be pretty bad, sometimes. I have not figured out which nights, or what the cooks look like, though I've tried. No turnover problems - when the good cook is there, everything's good, empty or not. Frustrating.
Renu Nakorn: Again, like Guelaguetza, major turnover problems. I think they especially give up towards the end of a night. Most of the meals I've had there at, say, 8:30 PM, suffer from a sort of listlessness. For example, once for sticky rice dessert late in the evening, they stuck it in a plastic bag in the microwave and it came out still shaped like the plastic bag. Tasted stale. Hard to get out there early enough, but it's worth it. I don't have enough data about which nights of the week are good there.
Ruen Pair: Surprisingly reliable at nighttimes, from 7:00 PM onwards. From medium-crowded to heavily-crowded, always good. The dead-time between lunch and dinner, though, they dish out some pretty stale-tasting stuff.
Samnaluang: This is the most frustrating one for me. Their noodles are consistently high quality, but broth quality varies WILDLY with no pattern I can find. Sometimes it tastes sparklingly fresh and pure and good, and sometimes it tastes like - I think Curt described it as "dirty dishwater", and that's about right. I have no idea what's responsible for this. They sure as hell go through enough broth in one day, and just being a day old doesn't make broth taste like that. Does anyone have a clue to a pattern?
El Gran Burrito: like most taco/burrito joints, quality is VERY linked to turnover. Best times to hit it up are weeknights around 7 or 8, and Thurs/Fri/Saturday from midnight on. Especially around 2 AM, when the clubs let out. It is always crowded, and at its best around about then. Carna asada quality is most turnover-dependent; al pastor medium-turnover dependent, and cabeza - a kind of stewy thing - very independent of turnover.
On the whole, all the Middle-Eastern places I frequent - Sunnin, Carousel, Elena's, Sahag's, Arax - are quite reliable. I think the kebab-and-fried-falafel orientation makes it pretty easy to keep some raw meat around and fire up a batch. Sunnin, in particular, has never had an off-moment in my memory. Zankou has an unpredictable cycle - there is a major difference between a chicken that's come out of the roaster 1 minute ago and 20 minutes, I think, mostly in crispness of skin - but I've sat and watched and haven't figured out any way to guarantee a fresh chicken. You takes your chances. Tarna quality varies, too, but this seems independent of crowd or time.
For some reason, most of my favorite Chinese restaurants in SGV are exceedingly reliable. The only time I had a substantially worse meal than I expected from a restaurant I knew in SGV was at Green Village the weekend after the review in the Weekly hit.
Any more data would be quite appreciated.