If the engine is starting and running normally again, with the crank breather isolated, then you may be able to reproduce the problem simply by removing the oil filler cap, if I'm not mistaken.
In a normally aspirated vehicle, the induction system will be operating with a depression in the manifold. In a turbo application, the manifold will be pressurised on boost, hence the check valve. The valve opening when off boost and closed so as not pressurise the crankcase.
In both cases, the breather system should be closed/sealed, if working correctly. Poor running then indicates that a fault exists leaving the system open, allowing too much unmetered air into the system. (Such as removing the oil filler cap). Mixture control can only compensate so much, before running out of adjustment.
One favourite place on the WR mentioned previously, and waxed lyrically about, was the rear crank seal. Being I5 engine, I'd imagine that this could still be a possibility.
From what I have read so far suggests that what has actually been done is the crankcase ventilation has been isolated from induction system; something that would also be done if investigating such issues. One item already attended to being a split hose. I'd be inclined to believe that there is still an opening somewhere.
ooh...maybe time for a pic
