I never bothered getting it fixed, it's still sitting in a box in pieces. You can get them from ebay for roughly £16, you might want to take it to a shop first and get them to diagnose it properly. If it is the synaptic then buy one from ebay and replace it yourself it'll be cheaper, unless your phone is still under warranty then you might as well send it in for repair.
Could be a problem with your synaptic cable as in hardware. When I dropped my skate in the sink I took it apart to dry it, but accidently severed the synaptic cable, I tried glueing it back together but it just mad the screen act eratic, very similar to what your describing.
I'll post a guide later, just need to dig out my digi cam. Tools you'll need for this endeavour : A set of small screw drivers, a flat piece of plastic for levering parts out ( I found the best tool for this to be a handle from a diy ice lolly kit :P ) hoover with brush attachment, surgeon gloves, toothbrush, anti static cloth or chamois leather and a clean workspace of a light colour for easy identification of missing parts etc.
Thanks man, I did pretty much this. Although as i was taking the screen off I snapped the synaptic thingymabob, so any hope of getting it working again are gone me thinks. I could always get a demmic skate off ebay and butcher it for parts :P