How do I get rid of 15 vertical lines going across my PC monitor?
It is a HP Pavilion desktop running Windows 7 enterprise.
HP Pavilions were notorious for a design defect that led to a successful class action suit in USA (eg), but pitiful recompense there (other than for the lawyers) and nothing at all elsewhere. Basically, the copper cooling strip was underdesigned and, I hear, in some cases there was no thermal jelly. (Yes, it is still a sore point as I don't want a doorstop for GBP5k.)
The symptoms you mention are too familiar.
However, there is some good news as IF this is your problem baking the board (eg) does sometimes work. Afterwards you should be OK because a reputable repair shop will upgrade the copper strip and apply suitable jelly.
Updating the BIOS may help (keeps fan running - whatever the specific cause, it is is likely to be heat related).