Monitors turning off

I have a very frustrating problem, in that it is very intermittent. I can
start the PC but within 5 to 10 minutes usually, the monitor will turn
itself off (the on off button will go amber....of flash...according to the
type of monitor) I have tried three monitors on the system and it has
happened with all three, over a period of about three months. The monitors
cannot be turned back on via the switches and powering off the PC and
restarting is the only way to get going again, and the machine will operate
fine for the rest of the day. One monitor gave the message ATTENTION 37K/OHZ
FREQUENCY IS OUT OF RANGE. This problem is very intermittent, in that it can
be fine for weeks, and then all of a sudden this problem returns for no
obvious reason. All power settings are turned off....that is, no sleep mode
or hibernating. PC is P4 running WinXP Home, GeForce4 MX440-8X.
Any ideas please.

Regards
Kaz