Several users have reported an issue in which keyboard, trackpad and/or mouse input will repeatedly stall after the update to Mac OS X 10.6.3. The issue is typified by the cursor freezing in place, or text input becoming suddenly interrupted then catching up a few seconds later.
As described by Apple Discussions poster gtufaro:
“So, I installed 10.6.3 using software update and was experiencing mouse freezing problems along with keyboard glitches (letters not typing, freezing). It seemed like it was mostly happening in Safari, but then I tried Chrome and the same problem.”
Delete errant kernel extensions. Early indications are that this issue could be related to a kernel extension (.kext) or kernel extension cache file. Kernel extensions are add-ons to the Mac OS X kernel, often designed to facilitate interaction with third-party devices or software. Specifically, some users have reported that deleting the following kernel extension:
then restarting may resolve the issue. PACE is part of an anti-piracy package installed by some software vendors.
Clear kernel caches. Some users have also reported at least temporary respite from this issue via clearing of carious caches (including kernel extension caches) with a tool such as AppleJack. To follow this routine, download AppleJack and install it, then restart your Mac in single-user mode by holding down the command and S keys at startup. At the prompt, type “applejack auto restart” (with no quotation marks) and press return
Reset PRAM/NVRAM. This fix has worked for previous instances of this issue. See this Apple Knowledge Base article for instructions for performing this procedure.
Remove third-party startup items. Third-party startup items that are incompatible with Mac OS X 10.6.3 may cause this issue. Look in the folder /Library/StartupItems/ for any third-party startup items and temporarily remove them then restart.’
Turn off keyboard light. This post suggests that, if you are using a MacBook or other portable Mac, turning off the keyboard light (by pressing the F8 key) may resolve the issue.
Downgrade to Mac OS X 10.6.2. As a last resort, you can downgrade to an earlier version of Snow Leopard using the “Archive and Install” feature from your Mac OS X installation disc as described here. Note that Apple made changes to “improve the reliability of certain third-party USB devices” under Mac OS X 10.6.3, which may be the cause of the issue.