Gianfranco,
While I as well have had problems with the battery indicator being reliable on the *istD, that may not be the problem.
I have seen the striping effect at ISO 1600 and 3200 with good batteries in the camera. There have been several threads in the Pentax SLR forum in dpreview about this and one belief is that the striping may be due to the camera trying to write to the compact flash while also reading image data from the sensor into the buffer. I do not know if that is true but the only time I have seen the striping is while shooting soccer games when I may shoot bursts of pictures. In casual shooting (i.e. single frame shooting) I have yet to see the striping. I don't know of anyone actually running an experiement to prove or disprove this theory but from the anectdotal evidence it is at least plausible. Another thought is that it occurs with some types of microdrives. Having never used one, I can't say.
This thread talks about the interference problem: http://forums.dpreview.com/forums/read.asp?forum=1036&message=11720512
This thread talks about the microdrive problem: http://forums.dpreview.com/forums/read.asp?forum=1036&message=11380387
Larry
