intermittant hesitation when running

On a 1988 528e. The engine seems to intermittantly back off power. Idle may even be rough when this happens. If I stop and turn off the key and restart, the engine runs fine till the next time. I found one end of the purge control valve disconnected. the end that must connect to the intake somewhere. Could this be responsible? How do find where it should be connected? I have looked all around the intake manifold but don't see an open nipple.