MacBook Pro Mid 2012 Flickers, Kernel Panic, CPU Panic, and Beep Thrice

After I upgraded my MacBook Pro to Catalina 10.15.5, it started to crashed from time to time and screen flickers. I shutdown my MacBook Pro and perform the resetting SMC and MVRAM / PRAM, when I turn it on sometimes it beeps 3 times and so on (Probably RAM issues). I open the MacBook Pro and removed the RAM and returned it to its slots, and it runs successfully. Later, after a few hours of use, the screen flickers and hang up. Sometimes it returns an error mostly, Kernel panic or CPU panic.

Every time all those issues occurs, I just removed the RAM and put it back again to its slot, and it will work fine, then later the screen flickers or the system crash. I checked its hardware problem through built-in diagnostic (Command + D) at start-up and find no issues.

It will run smoothly and no issues when I boot it on Safe Mode. Maybe the MacOS I installed is not fully supported with my Mid 2012 MacBook Pro or maybe something is wrong with the MacOS.

Somebody in here would like to help, suggest, and or recommend the possible fix of the above MacBook issues. Going to Apple support is quite expensive.