I might have a solution for this.
Make sure the desired resolution in XF86Config file is the only one listed.
Normally you have like "800x600" "1024x768" "1400x1050" etc
Make sure the line listing the resolution has only
"1400x1050" and nothing else also if there is a dot next to 1400x1050 MAKE SURE you remove it otherwise X will fail to start!
I've tried this before, it works on my PC
X starts automatically at 1400x1050 this way.