If you are using 4k or bigger footage, 8gig of VRAM is not enough. If it is getting worse as the computer heats up, then I suspect it is a hardware issue with the MBP which unfortunately have a bit of a reputation for having overheating issues.
You will get a lot better all round performance from an M chip Mac laptop including heat management and lower power consumption, plus Resolve has been updating with those chips in mind for Mac so all round performance is much better.