Hi. For the past fortnight, my Mac has been driving me utterly crazy. It's mouse-related but not caused by the mouse. The problem is this. If I drag something - say, moving a file, or highlighting over some text - it frequently 'loses grip' of it too early, even though I've still got my finger on the mouse button. This is super-annoying and only made worse by the fact that, while my finger is still held down, it sometimes regains the 'grip', so it becomes a new click and drag. Cue lots of mislaid emails and files, FTP mistakes, etc. Also, double-clicking to highlight a word ends up coming out as either a single-click or a triple-click (which highlights the whole line of text). It's intermittent but more often happening than not. I use a wired mouse on a 2010 MBP running the latest Yosemite, 10.10.5. - It's not the mouse. I've tried 3 of them, all wired. Same thing happens with each. - It's not the mouse mat - tried different surfaces. - It's not any one USB port - I've tried both ports on my MBP, and the keyboard ones too. - It's not any open applications - it happens when I just have Finder open. (No login items.) - Bluetooth is off. It's not that. - I've played around with the System Prefs > Mouse > Double click speed. High, medium and low. It still happens. - I've deleted the mouse prefs files like ~/Library/Preferences/com.apple.driver.AppleHIDMouse.plist and restarted. No joy. - I've checked 'Ignore built-in trackpad when mouse or wireless keyboard is present' in Accessibility Preferences. - I've verified & repaired permissions in Disk Utility - still happens after that. - I've verified disk in Disk Utility - it reports that my disk is ok. - It's not my Mac user account - I created a brand new user account, and it happens in that also. - I've booted in Safe Mode. The problem still occurs in that. - I've zapped the PRAM. - I've done an SMC reset. - I've run Apple Hardware Test (basic and extended) - no trouble found. - I've reinstalled Yosemite 10.10 from a thumbdrive (not a clean install) and it still happens. - If I switch from my USB wired mouse to a bluetooth mouse, the problem doesn't occur. Based on the above, where do you think the problem likely lies? Anyone had anything similar? Is there anything else I can try before doing a complete erase and install? Thanks.