Realtime is meant for well... a single real time task that is mission critical. Putting multiple tasks that aren't mission critical and are built on consumer level gear on a realtime priority setting is asking for trouble. Because they can't be realtime. Setting them to high priority is fine - you might want to try that but I think the fps limiting is going to be your best bet, aside from tinkering with drivers and settings or possibly adding another video card.

What is your CPU use sitting at when all are running? Can you check your GPU load? If it is at 100% then thats clearly the bottleneck.