The tasks each take a fair bit of data size. Quite simply, when running 6 tasks this data can't fit within the CPU, and it hits the ram hard. 2400 ram is pretty slow by current standards, but even if you went up to 3200 that wouldn't be enough to feed 6 tasks.
My suggestion would be to run these tasks multi-thread. I think 1 task using as many cores as you like is probably best, but you could also test 2 tasks at the same time, each using half the available cores. I don't think there is any way to change it for tasks already in progress using the web interface, so either endure it and let them finish, or abort and get some new ones with multi-thread set.