GRUB (dual boot)

[René]

hello all
dual boot always worked fine for the last one year.
since two days, when i boot into Windows (i do need it for special  
purpose), the machine gets lost in this boot process (does not show  
anything from Windows Boot), then reboots the machine automatically, and  
if i again choose Windows, this time all runs normal. it is reproduced  
everytime in this way, every second boot it does work fine. strange  
enough, isn't it?
regards,
René