Kernel_task running high right on from the boot

I am running macos Monterey v.12.6 on a MacBook Pro 2012 (9,1) with 8 GB RAM 240 GB SSD & 750 GB HDD with open-core legacy Patcher v. 1.5.0

Issue - Macbook suddenly shut down a few days back now will only turn on by pressing and holding the power button for more than 5 seconds and it will take forever to boot once booted the kernel_task goes up to 600-900% CPU usage.

I have tried : PRAM reset - crashes on the second restart SMC reset - tried but didn't find any noticeable difference (not sure if it did reset)

About to try : Reinstalling macOS and rolling back open-core legacy patcher to an older version.

If anyone has faced a similar problem and knows how to resolve this pls let me know.

Thanks in advance.