Shortly after the HTC Sensation was released, we covered a seemingly random issue with the touchscreen. Not all Sensation owners experienced this issue, and it's still relatively unknown what the exact cause of the problem is.
However, one of the theories formed on XDA is that it's a grounding issue on some devices. On others, though, the issue seems to be completely software related. If you can easily solve the problem by rebooting or a factory reset, then the fix outlined below will not work for you.