Although inactive plugins aren't the problem, you are still on the right track by looking at what's eating your system resources.
When the audio engine stops, it's because it couldn't feed data fast enough to the audio buffers. It'll ignore brief interruptions to the data stream (which you hear as clicks 'n pops) but if the buffer starvation lasts more than 250 ms the engine gives up and stops altogether. Going that long without filling the buffer is an indication of a severe CPU (or I/O) overload, which more often than not is caused by some process
outside of SONAR.
There is no one cause. A failing disk drive could do it. Insufficient memory, viruses, overactive background processes, a badly-written device driver, a defective network adapter - any of these things can cause the CPU to be distracted from audio tasks. High sample rates and/or small buffer sizes can exacerbate the problem, as can a large number of sampled instruments in a project.
The first place to start your diagnosis is SONAR's Performance meters. Watch these as the project plays. If the I/O meter shows a high value, your disk drive(s) may not be able to keep up, so that's one avenue for further investigation. If the CPU meter shows a high value, then the CPU may be having trouble keeping up. That's another avenue. If one core is at 100% and the others are low, that's yet another direction to investigate. If all three meters are showing low numbers and you're still getting dropouts or engine halts, then it's likely DPC latency from an overactive device driver.