i used to had this reset issue. several friends of mine too.
Reason for the failure:
1.Generic PSU has low (hold up) time.
2.High powered pc with low powerd UPS
lets say 17"crt dual core p4, dedicated GPU with 650va UPS
but 800 va is the very minimum in such case.
3.if branded PSU has reset issue then there is Faulty capacitor in the PSU( bad CAPs reduces hold up time significantly)
so what is hold up time?
its the time PSU can keep the pc running Without AC power.
usually 15 milisecond or 20ms
actually during this 15ms or 20ms time PC runs on the reserve
power of the capacitor.
so now u know what is the importance of capacitor.
so, if hold up time is low ( BAD CAPS) PSU cant hold power long enough before ups throws power so a gap takes place and thats when restart occurs.
Those who r having restart issue with UPS just change to a Better PSU which has good hold up time like (atleast 10ms)
ppl with generic PSU usually face this issue.
and to screen out the problem:
make sure ur pC can run without being reset in normal condition.
then u can blame PSU or UPS and take steps accordingly.
and if u can run ur pc without resettting 2nd time on ups( after the 1st reset during power failure) then its very likely that ur PSU is the source of problem.
hope this helps to make u understand the relation between PSU and UPS