I have had a similar problem - twice. Happened when I created a bus that had EQ on with Gloss (flat eq settings), G series, and the concrete limiter with it set only to limit to .1 The concrete limiter had no bars lit up however, so this may be different. It happened when I loaded X2a initially in Windows 7, then after I updated in Windows 8.
I stumbled onto turning off pro channel as the fix. Was baffeling for a few minutes, but not hard to work around. For our good friends at Cakewalk, if it is something I am doing wrong (and I know it could very well be), let us in on the change in habits required. :o)
The two projects were small - 8 channels of voice, rhythmn guitar, lead guitar, bass, accoustic - with a full SD3 set. Not very many effects - mostly pro channel tools, but I use a few go to tools from WAVES. What I am trying to say is, the PC handles this stuff easily. Put the CPU meters on in the toolbar and they weren't showing anything taxed.
My specs - a AMD A 64 6000+, Gigabyte MOBO, 8 gig Corsair ram, PC is never connected to internet - no virus checking on, 1 7200 500 gig hd for OS and Sonar, 1 7200 1tb drive for audio, win 8, MOTU 828 mkII - firewire (TI). I am going to swap out the CPU, MOBO and add another 8 gig in a few weeks..