Regarding a new instance, i think it does give you some flexibility in managing your resources but at the cost of overheads. I would concur with Allen that with the nature of the issue which you are having, multiple instances won't be a good place.

Also, before going that far, i understand that you have identified some of the intesive queries. Is it possible to restrict the resources being used by the intensive queries e.g by restricting the max degree of parallelism and see if that helps ? Also, is there any processing which can be moved at a different time ?