HTC Desire :: Bottom Half Of Screen Not Functioning Properly

I've just come to notice a problem with my desire, which I believe to be fairly long standing.For quite a long time I've been thinking that scrolling down lists/webpages/in apps etc has been quite stuttery, the wierd thing being that if you flick the list/page/whatever and let go (ie let the momentum take it) it's a lot smoother. I noticed today that when I'm scrolling with my finger on the top half of the screen it's actually really smooth, but as soon as my finger reaches the halfway point on the screen it starts to stutter. The stuttering isn't random, it seems like there's a grid of sensitivity on the screen, so it only catches up every 5mm or so (it feels like there's about 15 points of sensitivity horizontally on the screen).To test it I first downloaded the multitouch test app (as it purely reads from the driver where the touches are being detected and displays it on the screen). It still happened. I then downloaded a free drawing app, which seems to accentuate the problem, as when my finger reaches the midpoint of the screen it stops drawing altogether!I imagine it's probably a hardware problem as it's been happening for ages (only just recognised though) and during that time I've installed a debranded rom and then updated to froyo.Has anyone else noticed anything similar? I'll try and get a video on youtube to show the problem at some point. Any advice of things to try or should i just call T-Mobile support?