If you say you're using an old CRT, maybe it's not entirely compatible with all of the digital signals the graphic card sends to it, depending on how old that CRT actually is.
I remember that when I got a Pentium 4 many years ago and tried to connect an old CRT to it, which I used without problems on an old Pentium 100 previously, it didn't work with that Pentium 4. No display no matter the resolition, I specifically tested later. A new CRT I got after worked without problems, though.
Might have to do something with analogue / digital signal connection, I think older CRT's used analogue signals and somewhere at the year 2000 plus minus (maybe already 1998), graphic cards began using the digital signals, which became the standard for CRT monitors.
Edit: look
here for a short info.