350 Dps per dps role isn't much of a strain. Since you're getting a solid amount of stacks during add phase, it has to be the last phase/the phases in-between that shrekt your runs. Do you have everyone alive? Do you use melee lb between the mechanics? Do you have a lot of melee out of position? Is your SCH putting up dots? Does your bard need to sing a lot?

There's a lot that may cause the issue, but I think you should definitely use a parser to see what's the cause for the lack of dps after adds.