The problem is gone by its self and never back for weeks now. I can tell the reason was that I have been using the analog (while I am using the digital connection) mode for long enough (months) untill the monitor started to missup. The even when I search to find out the right timings for the digital connections, the monitor needed weeks to get back to normal. Maybe something else is going on, but who cares as long as the monitor is working again.
For the record, here are the correct timings of the monitor:
Modeline "digital:1680x1050@60" 119.0 1680 1728 1760 1840 1050 1053 1059 1080 -hsync +vsync
Modeline "analog:1680x1050@60" 146.25 1680 1784 1960 2240 1050 1053 1059 1089 -hsync +vsync
use the digital when the monitor is connected via DVI cable and use the analong modeline when connected via VGA cable. GTF calculator and xtinming do not report the right mode.