If it started right after you reconnected the monitor cable, there are 2 possibilities.
One, ESD when you were wrangling the cable might have damaged the card. It's not likely, but it is possible. I've received some shocks from touching the exposed parts of a monitor cable in the past.
Two, the card got jostled in its slot. This could also be a dust accumulation thing, where the jostling moved dust in/into the fan and some fell/into the slot. It could be overheating, it could be a minor shorting issue.
Try opening the case and removing and reseating the card, and if you can blow out the fan, the card's contacts, and the slot before you reseat.
The Jedi Maste