I've had the same problems with channel switching in Vista64 but this may be a workaround. Let me know how it works for you. My symptoms would be that the graph would screw up and not be rebuilt correctly I'm assuming. This results in a "no signal" even though my antenna was pointed directly at the station. It would just lock up when I moved the antenna and the video froze. Usually a reboot would fix it, but...this is a better solution so far for me.
Open the Digital Signal Quality meter (was an addon on my 1800 HVR). Before you change the channel adjust the slider to the "slowest" postion, or to the far right. Close the program. Change the channel and rotate your antenna to optimize the signal. Open the viewer then channel manager and the channel and should pop right in.
This is a workaround, but it's much quicker and easier than a reboot. This also works..(moving the slider to the right and then back to about a quarter of the slider range from the left if your video starts "stuttering" but still displays
Let me know if it works as it's a work in progress for me to get this thing working right.