Sorry, missed this post. I will try 275/0800 after we see what happens with 250/0750. My procedure lately is to put in freq/volt, save & apply, power cycle, then system/reboot. I added on the last step after just a power cycle would result in bad stats in Miner Status.
I used to have that issue (and always had to do an SSH reboot after a power cycle). Of late, though, it seems to have been fixed .... all I did once is before I rebooted via SSH, I first went into the System -> Start Up tab and stopped cgminer, then entered reboot in the SSH window. Now the stats are OK after a brutal power cycle .... (but I was called names when I mentioned the stats problem on this forum!)
Have you tried OC on an S3+ with factory thermal paste? The six I am working with are untouched. So if it is a chip-level thermal problem that is not reported to the sw, then I guess that could explain the difference here.
I did OC one S3+ before I redid the paste which worked OK at 262.5 (which was the only "good" freq I had at the time). I now redo all the S3's I get by, at the very least, putting heat-pads on the chips, so for now all have been "modded" if you like.