Intermittent boot problem

Power failures, surges, lightning, etc can cause many problems.  Get the UPS ASAP as it will minimize the AC power line problems (but won't do anything for lightning).  If you have one of the newer "Active PFC" power supplies you MAY need a "true sine wave" output UPS (for when on the UPS battery backup).  Most of the "consumer" and even some of the commercial UPS systems are a modified sine wave and the switching time is too slow for Active PFC power supplies.

If you have more than one hard drive connected, disconnect all except the boot drive and see if that makes a difference.  Considering the M/B has been replaced that should eliminate it.  Has the other hardware such as Video Card and/or sound cards been replaced?  The video card only uses the basic VGA mode during POST (boot) and when in Safe Mode, however when the regular Windows mode starts it brings in the video drivers for higher modes and that can be an issue.   Has the boot hard drive been replaced?   Diagnostics are great, but if a device is intermittent unless it fails during testing the diagnostics will not show anything. 

Thanks guys, will try all of the above - and that's the first time I've heard that 1600 RAM is no good, will give it a go with 1066.

Did your tech replace the RAM? You mention that he replaced every component and that you've already dealt with bad RAM, but you may have put more bad RAM back into the machine. As Scott mentions, I don't think the speed is an issue, though. Get a good RAM tester and test the RAM thoroughly. Also, try booting with just one stick (or the bare minimum possible) at a time in the system.

Another test would be to boot to Safe Mode, remove all the "Display adapters" entries and reboot. As fireberd suggests, this could be adapter-related (though since you've replaced everything else, it's a bit of a long-shot). You don't mention if you have a dedicated video card; if you do, remove it and try booting. You can also have Windows boot with VGA mode drivers; it's an option if you hit F8 while it's booting. At any rate, update your video drivers.

If your power is that bad where you are, get a power conditioner in addition to a UPS--or get a better UPS. A UPS will help if the power dips or fails, but it won't "clean" the power. Furman has relatively inexpensive conditioners that will also help with your other electronic gear. APC's more expensive UPSes will do this, too. They start at around $120. Think of it as "insurance" for the money you've already put into this stuff.

One more thing: I'm working on a laptop with Win 7 that's having a similar problem: wouldn't boot to Windows. After saving the data, reformatting and re-installing, it still would boot to Safe Mode, but not Windows. Having nothing to lose, I reformatted and re-installed again. Works fine, now. Go figure.

UPDATE: The laptop I'm working on failed to boot again. Booting to Safe Mode and removing the display adapter (and its associated files) allowed Windows 7 to boot "normally". In this case, the on-board video adapter may be failing, or the drivers could just be bad. Your mileage may vary, of course.