strange prob with ups & monitor

[hellgate]

my frnd has bought a new pc.
the config is:
X4 9850 @ 2.9GHz
MSI K9A2-GM
3GB DDR2 667
320GB hdd
Palit HD 4850 512MB (Catalyst 8.9 installed)
HP dvd i1035 dvd rw drive
Glacial Stars GA-500W psu
HP w1907 monitor (connected with a dvi-d-sub adapter and d-sub cable)
HP 800VA ups

now the prob:
1st he had got a Microtek 800VA Double battery ups, which ran fine 4 a few days but after that theres a strange prob.whenever theres a pwr cut the ups goes into battery backup and the monitor switches off showing Monitor going to sleep and the amber clr led glows,but the system conitunes to run fine.
after i reconnected this prob was solved and now monitor doesnt switch off but when the pwr comes back and the ups switches from battery to mains the monitor switches off again.
so he went and got the ups cahnged with a HP 800va.at 1st this prob was full solved but then again the same prob came back.
now i cant understand why this is happening.

i dont think its a psu related prob cuz even on full load the system stays completely stable.also i dont think 2 upss wud be faulty.

now some1 plz help me solve this prob.Os i Vista x86 Ultimate.

Thnx.

[Raccoon]

I'v had a huge amt of UPS problems.... and all I can say is that they all vanished the day I decided to buy my APC.