CPU usage super high but no apps running

Hi All,

My Mac is pretty old (MacBook Pro (15-inch, Early 2011)) running OS X El Capitan but has always been working fine until recently. All of a sudden it started running really slowly, to the point that even moving the mouse wasnt smooth, it was jolty and glitchy. I had a friend have a look at it and saw that the CPU was spiking despite no apps running, was showing the dreaded kernel_task being the main culprit. I've read a few threads today and tried resetting my SMC which didn't seem to do anything, also tried running in safe mode and the problem seems to go away which would suggest its a 3rd party program. I also tried checking the kextstat for an obvious program to remove, but couldn't see anything that wasn't an apple one. PLEASE HELP!