Thank you all for an interesting discussion and the link to Peter's Thread. I have now tried everything I can think of and nothing works. Strange about the rapidly pulsating top display, which Peter did not experience.
So it's back to CRIS in October for a fix, which will probably cost me about half a K-x. Problem is, I really like the size and handling of the K-20 - even better than the K-7, which I also like.
Btw, for further information, I should disclose that this same camera has been to CRIS once before for a totally different and even stranger problem, which I posted on dpreview here (I was following but not posting on pentaxforums at that time):
Need help. What's this? Chipped sensor? Dead pixels?: Pentax SLR Talk Forum: Digital Photography Review
I wonder if the CRIS people made some kind of mistake when reassembling the camera after replacing the sensor. I'm not suggesting it; just wondering to myself.
Anyway, unlike Peter, I am finding this thing rather tiresome, as the K20 is my favourite camera and the DA17-70 + DA*50-135 is my favourite zoom combination.
Mostly, I tend to use primes, however, and the K20d is still good for those ... and then there is always my K-7 and K10d at a pinch.