As I said on EcProjects video, my DS1074Z from spring this year does not suffer the 5us trigger problem.
I measured a 25MHz oscillator, so if it would be a beat frequency you may see the same thing at 4us (-20% period).. nope nothing there.
I still run v2 firmware (SP5). That firmware version doesn't show the board version, but I assume it's a very early-on model.
It seems like the AC coupling issue is also present on this older firmware version.
The LFR does the same thing.
In fact, I think I can do better. If I put the scope channel to AC coupling as well (so it triggers when it zero-crosses), I get this weird artifact:

I'm pretty darn sure this oscillator & board is OK; because it runs a 100Mbit ethernet phy at full speed without any hiccups. (And also it shows no sign of trouble on all DC coupling)
edit: The internal function generator does the same, so it's in the scope AC triggering. The artifact changes with frequency quite drastically, but even at 5MHz the triggering is very jitterish.
I have recorded the frames using the record function. The "jitter" on this image repeats every 8 cycles (coincidence 2^3?). The first 3 frames are clean. On the 4th the waveform jumps 3ns to the left and gets progressively worse over 5-7 frames. On the 8th it jumps back. This repeats on and on..
I also noted that the LFR/AC does not have a trigger level pointer next to the screen side, but it does show a "voltage" in the right hand corner. What is that all about? It seems like the AC-only triggering is off by quite a bit.