EVGA G2 850W

in idle
 
1) Are my 5VSB at normal value? I found this:
+5Vsb tolerance
±5% - from +4.75V to +5.25V
Mine is 5.261V
 
2) Is it normal that in load (AIDA64 stress test - everything is on 100% usage) voltages varies? example: from 3.328 to 3.313,...

Of course, if an EVGA power supply is running out of spec, it should be replaced if it's under warranty.
 
However, as Sajin noted, software readings can be pretty inaccurate.  I would suggest picking up a cheap multimeter and use it to read the power supply under load to verify that the power supply is running out of spec.  It's also a good idea to check the voltage readings in your motherboard BIOS as a backup.
 
 

TeslaPower
What problems would I encounter if it is really out of spec?

Computer would cut off under load.