Hi Stefan. I don't know what the problem is but I would check out this page
http://www.weasner.com/etx/techtips.html on Mike Weasner's site, and read the Performance Enhancements section. I would firstly try training and calibrating the drives and then upgrade the firmware if it's not the latest version. if that doesn't solve the problem then wading through the vast amount of information on the linked page will hopefully resolve the issue for you.
I have an ETX 105 and I found it to be a steep learning curve getting it to function as advertised but when you get them sorted they're great.
PM me if you keep having trouble and Ill render such assistance as I'm able to.
