Seems the SMC & PRAM & HDD is fine, there might be some problem with your Logic Board. Try to run Apple Hardware test. Follow the below steps to run Apple Hardware Test (AHT):
You need to disconnect all external devices connected to Mac before running Apple Hardware Test.
* Place the Mac on Hard or ventilated surface to avoid any kind of error
* Restart The Mac and Press & Hold the D key on your keyboard
* Apple Diagnostics starts automatically
* Select the language you want to use & click the right arrow
* Click the test button to start testing, press T
* Review your test results after completion
* Again restart the system to quit Apple Hardware Test