Over the last few months my Mac has developed a worrying habit. Within the first few minutes of starting it up (perhaps on average about 50% of the time) it will completely lock up. This may happen at the Log In screen (if I start up the Mac & then leave it for a while) or during normal use. Often the first symptom is a very high pitched (but quiet) whining noise that seems to come from the loud speaker on the front of the Mac. The pointer may freeze at this point, or it may still be moveable for 5 or 10 seconds before it freezes. It sometimes turns into the spinning beach ball during this. Once locked up the only way I can restart the Mac is to hold down the power button on the front for a few seconds to completely reboot the machine. Once the Mac has restarted, it usually behaves normally, almost always for the rest of the day. The initial lock up & resulting restart only normally seem to happen the first time I use the Mac that day. The only peripherals attached to the Mac (apart from the display, keyboard & mouse) are an ADSL modem, a USB printer and a pair of Apple Pro speakers, and this setup hasn't changed since long before the problems started, so I'm confident that I can discount the peripherals causing problems. I doubt that unplugging the speakers, for example, would have any effect. I've run Disc Utility, OnyX and DiscWarrior without anything major cropping up. My instincts (I've been troubleshooting Mac problems for 16 years) tell me that I have a fundamental hardware problem, possibly with one of the 4 RAM DIMMs installed. The RAM configuration is shown in the attached screen grab. I'm considering removing one DIMM, running with 1.5GB of RAM rather than 2GB for a while, and repeating with a different DIMM removed each time until I can hopefully isolate the dodgy DIMM. Do people feel this is a sensible approach, or should I try something else first? Many thanks.