Update your post to include full system hardware specs and OS information.
Monitors: make, model, connectivity (DP, HDMI, DVI, VGA...)
PSU: make, model, wattage, age, condition?
"All monitors"?
Swap monitors around if at all possible. Determine if the problem follows one particular monitor or stays with a particular video port/cable.
Go back into the case and take another look at all cables and card seatings. Maybe something was bumped loose when the new RAM was installed.