[Charlie]

Hello all,
I've been musing on a small mod for one of my computers but my current abilities for designing circuits are so rudimentary even this is proving too hard...

Here's the situation:
-Video chip takes one of three potential input clocks: 24, 25ish, and 36mhz depending on the intended output - selected by the OS.
-The 36mhz input is shared with the memory clock & I've been, um, overclcoking it a bit.
-The poor video chip isn't coping with the rather higher frequencies I'm driving the memory so in modes that use the shared '36mhz' clock I'm getting lots of graphic corruption.

Plan 'A'
There's a link on the motherboard which if removed will cut the input clock to the video chip so it should be a simple task to plumb in a 36mhz crystal @ that point...
...trouble is the modes that rely on the 24 & 25ish mhz clocks will be driven @ 36mhz too which won't work so well...
...Sound is provided by the video chip so if the OS doesn't know what frequency is being used one gets Pinky & Perky!

Plan 'B'
Knock up a circuit that takes two inputs:
-My 36mhz 'reference' clock.
-The OS selected 24, 25ish, 36++mhz from the computer.
Logic:
-Pass through any frequency less than the 'reference' clock
-Replace any frequency > the 'reference' clock with said 'reference' clock.

Plan 'B' would be MUCH better - can any kind soul point me in the direction of suggested further reading / logic to use?

Many Thanks

P.S.
Another option would be locate the point on the motherboard where frequency selection is made for the 36mhz clock and split it - one to memory & one to video...
...sadly no schematics available.

*Edit*
The video chip in question is very tolerant of 'skew' when changing between driving clocks so a straight switch over without consideration of signal synchronisation is fine.