@Swisso
My 7455 933MHz chip running at 1.0 GHz has been running like that for at least two years now. Before that it ran at Eyetech's original 800 MHz speed.
No problems with my system, but that is because I replaced my heatsink with a Zalman northbridge cooler and a homemade duct with an 80 mm fan in just a few months aftr I got the A1 (before even trying to overclock it). My CPUDocky is showing 131?F right now (about 55?C if I did my math right), and the Silverstone temp monitor in my top 5.25" drive bay is showing 120?F (the probe is measuring the temp of the heatsink above the CPU core, probe stuck between the fins with the tip touching the bottom of the heatsink). Note my CPUDocky is not calibrated, so this temp is likely off a bit. However, since the measured temp with an external probe is only a few degrees cooler on the fin side of the heatsink, I'm thinking the CPU's internal temp circuit is working fairly close to what it should be.
I suppose I could get it to run cooler, but this runs nearly silent. Much better than with the crappy fan/heatsink that Eyetech put on it originally.
Anyway, I mainly wanted to point out that I do not believe the reliability issue is the SPU itself, but likely the assembly of the CPU module and/or other parts on it. These PowerPC CPUs are originally designed for embedded operation, usually in environments that are not necessarily computer-friendly. I would have to say that Motorola/Freescale/IBM have likely "over-engineered" the CPU a bit so that they would work more reliably in those embedded environments.
And we know that Eyetech did not use the correct reference documentation for the CPU they actually used on most G4 setups, as they used an inadequate cooling solution and did not se the voltage to the correct one for the CPU. The 933 MHz 7455 that I have, along with many others, needed 1.79 volts, no the 1.6 they shipped it with, so I'm surprised any of them worked at all!