Old sgminers and cgminer 3.7.2 did indeed have leaks on the GPU, but I fixed them a few months ago when I did the "hot-switching" between Scrypt and Scrypt-N. So any semi-recent build of sgminer should not have leaks on the GPU, it is probably something else.
Indeed. I've had similar problems many times with sph-sgminer, but never with a recent sgminer. After running sph-sgminer for X11, hashrate would always drop until reboot.
It seems the algorithm vs kernel changes fixed this for sgminer. Althought I can't say I have tested the v5 branch extensively yet, I haven't seen this issue crop up (and it happened every time with sph-sgminer).