it seems like you have 2 extra amps than you actually need to run the card.
though this only has a 20pin instead of the 24pin most PSU has nowadays.
If you get a new PSU, try to get one with 2 PCI-E connectors.
it either seems like theres something wrong with your computer (hardware or software) or that something is bottlenecking.
do you have a friend that has a good PSU?