Also worth noting if your shell has been banged enough to cause the malfunction. That's what happened to my 2008 MBP. I had thought it was a battery bulge because I did in fact have a slightly bulging battery, and I could temporarily alleviate the trackpad iffiness by applying pressure to the sides of the pad (which are above the battery) But replacing it didn't make my intermittent issues go away. I also tried adding a paper strip above the cable to add pressure to the connection. No go. Turned out that I had knocked it once too many and the case was fairly bent off the mark. So it required a replacement shell, which includes the trackpad. Got a refurbed part installed very reasonably (not by Apple, at Dr. Brendan's Mac Repair in Brooklyn, only a few blocks away : ) and that was the only thing that got me back in business.