Firstly, if the motherboard has 4 RAM slots, are you using slot 2 and 4 (so that it goes empty, RAM, empty, RAM)? If not, change it so that it is like that. Also, have you tried uninstalling you graphics drivers, and then installing the newest available from Nvidia? You can get them from https://www.geforce.co.uk/drivers/results/148635.
Lastly, what is the manufacturer, model and wattage of your PSU?