Just beacuse the Wattage is adequate does not mean that the power supply is keeping steady volts or amps on the rails.
From past experience of a lot os psu's, you defo get what you pay for.
Cheap £20 - £40 650w psus generally are not up to the task.
The PSU is the only part of a pc i would defo suggest that you spend decent money on, you will not regret it
