2009 27" iMac reboot loop

Hi there! Was wondering if anyone knows what would be happening with my iMac, every time I turn it on it keeps restarting for a few times then will go to desktop but will reboot once every 15 mins to half an hour, anyone know what might be happening with it? It's a 2009 core i5 27" iMac,

Cheers

Welcome to the Mac-Forums.

Spontaneous rebooting is generally caused by a hardware problem. Here are the three most likely suspects:

1. Bad memory module

2. Bad power supply

3. Overheating

You can check the memory fairly easy by downloading a memory checking application and you can also monitor the temperatures in your iMac by using iStat Pro. Checking the power supply is more difficult and would probably be best letting Apple check it for you.

I'd do an SMC Reset, if not a "PRAM" reset as well. just in case:
https://support.apple.com/en-ca/HT201295

Did the problem just recently start?