Try running the Apple Diagnostics to see if any hardware issues are detected.
Disconnect all external devices in case one of them is causing a problem.
Try booting into Safe Mode to see if the laptop has the same issue. If Safe Mode works, then maybe you have an issue with some third party software. Run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper. If you give EtreCheck "Full Disk Access", then a summary of the recent system logs will also be included which may contain important clues.
Boot into Recovery Mode/Internet Recovery Mode or into a bootable macOS USB installer to run Disk Utility First Aid on the hidden Container. You may need to click "View" within Disk Utility and select "Show All Devices" so that the hidden Container appears on the left pane of Disk Utility. Even if First Aid says everything is "Ok" click "Show Details" and manually scroll back through the report to see if any unfixed errors are listed. If there are any unfixed errors listed, then you will need to erase the whole physical drive before restoring from a backup or clone.
A 15" Apple laptop is also known to have GPU issues. You may want to try forcing the laptop to sue the Intel GPU instead of the discrete GPU by using the gfxCardStatus app:
https://gfx.io
It also never hurts to perform an SMC Reset or a PRAM Reset (hold the PRAM Reset for at least three chimes).