Thanks...the more I have been using the up and down buttons the more I suspect the selection handling is seriously bugged. I think my previous description may be too simple for a reproduction of the grid selection problems, but I am glad a dev is possibly willing to look at it.
Today I was selecting several files and clicked the up and down buttons to rearrange the download priority. Every time after shifting the priority about 8 files up or down the selection would just disappear and on one occasion the selection would have extra files added to it! Houston we have a problem

Cheers guys.

edit: Ok gluewurm, danke shon