I quite often notice that static pictures with intricate detail suffer from a regular (about once per second) cycle where the detail goes fuzzy, then back to really clear.

It's a bit like what happens when a progressive jpeg is loading - sort of grainy on the details that then gets progressively better before the whole cycle starts again.

I suspect that it happens most of the time, it's just not as obvious unless the picture is static and detailed.

This is with a humax foxsat hdr, and I see it most on BBC4 with the arty programmes, but I've seen it on most other channels as well.

I'm using a 100" FP screen (Sanyo Z5).

Is this artifacts in the transmission stream, or problems with the humax, or with the projector?

Yep, that's the way MPEG works.
Every so often you get an I-Frame that is the complete image. This is then followed by P & B frames that update this image.
The interval between I-Frames is up to the encoder.
If you have x bits-per-second channel capacity then the I-Frame must fit in x/50 bits (on PAL) and this is effectively a JPEG image. The following P & B Frames also get x/50 bits to update the image to reflect how it has changed. With a static image this capacity can instead be used to provide additional detail which is what you are seeing.

Thanks Badvok, my fears confirmed. this can be really irritating at time. Just had a look at a mythtv recording as well, and it looks exactly the same. Still at least it's not something at my end.