Why does the CPU max out when I try to use Tascam US-122L with Mainstage?

So, I bought a Tascam US-122L to connect my MIDI keyboard to my mac, but unfortunately the mac won't recognise it as a midi device. Anyway, to get round that, I connected the MIDI keyboard directly to the mac using a MIDI->USB cable, and then plugged the Tascam in as well. When I loaded Mainstage 2, I changed Mainstage's 'Audio Out' setting to the US-122L (I want to use it for a gig and the standard headphone port on the mac doesn't output enough). Immediately the CPU usage went to red and stayed there. I played for a bit and it seemed fine other than the CPU being maxed out, but after about 20 minutes I got an error message saying 'You need to shut your computer down'.

What did I do wrong, and how can I solve this issue?