I attached a minimal file that shows the problem It works perfectly with Lyx 1.2. with dvi and pdf.
I cannot believe, please send the 1.2 LyX, log and tex file.
\papersize Custom
why custom and not a4paper? Layout->Document->Paper
\topmargin 15mm \rightmargin 20mm \bottommargin 5mm
it is impossible to place a pagenumber in a 5mm footer!
change your paper size to a4, add some more space to the top and bottom margin, then everything works well.
by the way: do not use the font setting from inside LyX, it is deprecated. Write instead in the preamble:
\usepackage[scaled=0.92]{helvet}
\usepackage{mathptmx}
\usepackage{courier}% luximono looks nicer hereHerbert
