At times it heats water other times it doesn't is it thermostat or elements?

Intermittent hot water can be caused by several things, some which are not part of the water heater, such as timers on the power supply, or scale buildup in single-lever fixtures. There may be troubleshooting section of the manual provided with your water heater. If you are not able to troubleshoot using an electrical tester, it's best to contact a qualified tradesperson in your area who is familiar with water heaters.