Well, this is embarrassing.
Please ignore my previous post. There's nothing wrong with KDFW-DT 35 or KDFI-DT 36. My reception was being degraded by interference, which was also evident on (analog) KJJM 34. Apparently it started Saturday on channel 36 and slowly dropped in frequency until 36 was OK again but 34 and 35 were unwatchable. Naturally this had to happen just as I was installing a new VHF-Lo antenna - a coincidence which led me on a few wild-goose chases.
I finally homed in on the interference source by using a portable TV tuned to channel 34 and a small directional 2-bay UHF antenna. The problem turned out to be a recharger for a couple of walkie-talkies. Once I unplugged it, the interference cleared up. I plugged it back in and the interference came right back.
I reseated the walkie-talkies in the recharger, and then could plug everything back in without the interference coming back. Perhaps one of the walkie-talkies wasn't seated properly and was getting weak recharge current, and somehow that caused it to radiate way outside its normal frequency band. Who knows?
It occurs to me that this illustrates a problem with our much-vaunted digital TV technology. With an analog signal, it's pretty easy to distinguish between a weak signal and interference. But with digital, the result is exactly the same: no lock. And even if you do suspect interference, there's no homing in on the source by turning an antenna to null it out. It's worrisome that a station can be blocked by something as simple as a malfunctioning walkie-talkie several yards behind a directional antenna.