This issue is usually a poorly designed cooling duct that is blowing too much on the nozzle or heater block causing the temperature to drop. First thing to try if you don't have one is a silicone sock on the heater block. If you already have a sock on the heater block then you could try a higher wattage heater cartridge.
As for a power issue, easiest way to check that is with a volt meter on the voltage leads where they connect to the control board when you have everything turned on (heated bed, hotend, etc). Measure the voltage before everything is on to get a base line and then with everything on and if it drops I would say more than .5 volts then you may need a higher wattage power supply.