I do not use any automatic frequency changers, i have a bunch of shortcuts on my desktop to switch between 24p, 50p and 60p.
My 24p resolution is pretty standard, just created a resolution for 24Hz, switched to manual timings, and changed the refresh rate to 23.978 (yes .978 - that gives me the closest output to 23.976), leaving everything else default. This worked fine for me, of course this highly depends on your AVR and your TV, so YMMV.
If your rate is too high, try to reduce it slightly, if its too low, increase it. You can additionally try to tweak the "total vertical pixels" number, but change those one point at a time, because it may have a significant effect.
I have a GTS450, btw.