We had what sounds like the same issue, slow speeds or loss of DNS connectivity after connecting any dual aggregated pairs on our MS425 stack. Unplugging one fiber link would resolve the issue. We tried rebooting the stack but that did not seem to resolve. We didn't try deleting the LACP configuration. Downgrading to firmware 11.31 seems to have resolved the issue for us, we've not had it happen again. We're waiting until seeing something about it in a firmware release before upgrading.
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