The 1084 was a good CGA monitor for the C=128, but for the C=64 any NTSC
analog video monitor will do (using composite video combining luminance
and chrominance information). Also, turning up the brightness / contrast
of your monitor may increase the visibility of retrace lines near the top
of the screen.
Do the lines disappear when you turn the brightness down?
If you have to turn it up to max brightness to view the monitor, your CRT
could be failing. The cost of replacing the CRT could be more than that of
a used PC.
On the other hand, most cheap analog color TV's will have RCA-type sockets
that will fit the composite video / audio cable provided with the C=64.
The yellow connector is the composite output of your Commodore computer.



If it were a CRT problem the retrace lines would fill the screen. When it's
just a few lines at the top, the problem is a capacitor at least 99% of the
time.