Well, I dismounted the cpu unit and replaced cpu 1 (the one below assuming G5 stands vertically) since ASD diagnostic showed an error checking AD 7417 AD1 sensor. Assembled cpu unit, reseated and power on. After the chimes sound G5 booted MorphOS and I immediately noticed the difference: low temperature (started from 66 C degrees to 82) and quiet fans. I booted the ASD cd and checked for thermal calibration wich reporting an error message explaining that wrong values were found and reccomending to perform calibration. Did it and both cpu passed. Reboot and MorphOS runs ok with temperature between 80.9 and 81.1 and fans really quiet. I'm really satisfied! Let's check how it will go next days.
@Intuition:
Sorry to read your post only now but if you should encounter any problem downloading 2.5.8 let me know if I coul help in any way.