PS3 fat that freezes randomly

I bought this PS3 fat (Refurbished) from
GameStop back in 2013, I know, huge mistake to buy there. A few weeks after I got it it would start freezing, like full system lock up, would require switching it off and on, and it can happen in 8 hrs or in 5 minutes, the more I use it, the faster it happens. I opened it up in 2014 and replaced all the thermal pads and added thermal paste where it was supposed to be because I figured it was overheating. The fan was upgraded to the 19 blade fan prior to me getting it, I even modded the casing with holes at the fan with wire screening to allow for more airflow, no success. I’ve done full system restores and 2 different HDs. Everything I’ve read said it’s gotta be over heating but sometimes it doesn’t seem like it’s on long enough to over heat. Anyone have any direction they can point me in as to what might be the problem?

Try to over heating up on GPU and CPU with hot air gun for 10minutes

What does that end up doing?