MBP 17" boots in safe mode but crashes when shift is released

Hello!

So my MBP wouldn't boot up or charge at all. After doing an SMC reset the charger changed from green to orange and the battery indicator then started to indicate power.

After this, i left my laptop to charger overnight and it would power up. The grey apple icon would appear and then the machine would crash.

I then launched into safe mode, holding shift, and the progress bar would come up and I would let go of shift and the machine would crash. I then did it again but put a weight on the shift key (as it's a long load to keep your finger on the button) and it went to login screen. I took the weight off to then enter my password and it crashed again after a few seconds.

Doing the process three times I pulled the weight up, entered my password quickly and replaced the weight, and it logged in! I thought it MUST be safe to release the shift key? It wasn't! After 15 seconds the machine crashed.

Does anyone have any ideas on what the problem is? I can't think of anything else I can do.