I don't really understand why this would be a multitasking issue (although I agree with others in this thread, multitask sucks when it reloads everything)
Usually when this problem occurs, it happens when I pull my phone out of my pocket after its been there awhile. I unlock the screen and type into google to look something up, and then nothing happens.
Also, I have a standard wallpaper, and only 8 apps and the HTC clock/weather widget thing on my homepage.
And I did as you said and cleared my cache and moved my 11 apps to my phone storage. (I hardly have anything at all on my phone)