Any codes stored in the computer? If not, I would start by unplugging the mass airflow sensor (you will likely throw a check engine light doing this, so be prepared to reset) and see if it idles normally. My Buick 'hunted' for idle & i had to keep my foot on the gas to keep it running when its MAF went; the Volvo just wouldn't stay running, so I guess any number of weird occurrences can happen. You may also want to check for vacuum leaks, turbo lines (if it's a turbo), etc. When's the last time you changed spark plugs?