The original PIII still used the .25 micron (and I think even had the external L2 caches on the package (2 memory chips beside the core on the slot1 cartridge). These things were only released to 600Mhz and this was a long stretch. Very few can make it to 650 (even with water cooling and peltiers). They just were not made to take that speed. Even 550Mhz is a good overclock for most. There are a few things that could cause it to crash.
The easiest is the voltage. As you increase the speed you sometimes need to up the voltage a tiny bit. Try a very small amount at a time. This tends to generate more heat so you want to keep it to a minimum.
The second is heat. These things do run a bit hot. You can either add a better fan (or fans) to you heatsink or get a better heatsink. There are still some slot1 coolers floating around. The other thing is case cooling. If you run with your case open and a fan blowing in an the lock-ups go away, then you can add an extra case fan or two.
The third thing could be a system componant. One of the problems of overclocking with the bus speed is that you are overclocking just about everything in your computer. At 122Mhz, you are looking at 41MHz PCI and an 81MHz AGP bus. Most things can handle this, but the chances that one componant has intermitant problems at this speed is pretty likely. The only thing you can do is either start pulling and/or replacing parts or turn back the bus speed a few notches. Memory is also a main thing to fail at higher bus speeds. Sometimes you can set the memory settings in the system BIOS to failsafe and get the memory to run a bit faster, but you might actually slow down your system doing so. Memory tweeks can give you quite a bit of performance.
The COMPUTER is your FRIEND!
Happiness is manditory.
AMD Phenom II x4 945 3Ghz | ASUS M4A77TD | 2X WD 1TB SATA 2 hard drive | 2x2GB Corsair XMS3 | nVidia GeForce 8800 GTS | ATI TV Wonder Theater Pro 550 | Antec P-160 case | Antec 650w Earth Watts | LG Blu-ray Super Drive | LG DVD RW | Windows 7 Pro