Thanks, and that's also my first idea: recent JCEs are faster but support a lot less to mine on 99% available memory. The trick is that the b5 and b6 no longer need to use all memory to get fast, I tried to configure my 4G cards exactly like my 2G (with about 1900M memory allocated) and they mined fast. Increasing the 4G cards load over 2G gave very little extra speed at the cost of a very unstable hashrate.

Maybe the fix is just to warm users to update their older config and use less memory. I'm afraid a lot of users just updated their binary keeping the old config on 8G with maximum memory load, as it was the rule on previous versions, and is still on other miners like Cast or SRB.

To all who have the problem: configure your 8G card like if it was a 6G, leaving free vram on it and retry. I'd expect to get -1% speed and a very more stable hashrate.
I remember this rule from older Claymore 9 with mode -a 1: it provided +2% extra perf with +20% power consumption, 100% memory usage and random crashes. Not a good deal.