I'm not saying it will fix anything, but your custom settings are not correct according to the user manual at
http://www.planar.com/monitors/pdf/Manual-PL191M.pdf .
And even that manual had conflicting info as to the settings. According to the preset configs and the spec page, you should be able to set:
vert to 50.00 - 75.06 (hz)
horz to 31.47 - 79.80 (khz)
Please read the manual again and see if you agree. You can then try it. As always with X settings, out of range settings can blow the monitor. If you really really want to be safe, send an email with your requested settings and see if you get a response from the people at dell or planar.
Another program you can try is 'xvidtune', but I've always had problems getting the settings to 'stick' with that program.
You can also look at the /var/log/XFree86.0.log file and see if there is anything strange in there.
Please post your results so that you may help others.