FLC OF A C560 EXPERIENCES AN ALT EXCURSION DURING A MOMENTARY LAPSE OF VIGILANCE AND DISTR.

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ENRTE TO LAS; FO LEFT COCKPIT MOMENTARILY. CAPT WAS PF DURING THIS TIME. CAPT RECEIVED ATC INSTRUCTIONS TO DSND TO FL310. HE THEN SELECTED THE FLT DIRECTOR TO HIS SIDE AND PROCEEDED. ATC THEN CLRED TO CROWE INTXN ON THE RWY 25L LOC. CAPT WAS UNABLE TO LOCATE CROWE AND THE FO WAS RETURNING TO THE COCKPIT. CAPT TOLD FO THAT HE HAD THE FLT DIRECTOR ON HIS SIDE AND THEN SWITCHED IT BACK TO THE FO. THIS ACTION CLRS THE AUTOPLT. THE FO THEN RE-SELECTED NAV ON THE MODE CTLR. AT THIS POINT THE FO PROCEEDED TO HELP THE CAPT FIND CROWE INTXN. BOTH PLTS WERE OF THE MIND THAT THE AUTOPLT WAS 'FLYING' THE AIRPLANE. HOWEVER A MOMENT LATER WE BOTH NOTICED THAT THE AIRPLANE WAS 600- 700 FT LOW. WE IMMEDIATELY CLBED BACK TO FL310. AT THIS TIME ATC ASKED US TO VERIFY OUR ASSIGNED ALT AND THEN CLRED US DOWN TO FL240. THE BREAKDOWN WAS WHEN THE FLT DIRECTOR WAS PUT BACK TO THE R SIDE. THE FO ONLY SELECTED 'NAV' AND NOT 'ALT.' THE FO THEN DIVERTED HIS ATTN TO HELPING THE CAPT FIND 'CROWE.' THIS BECAME A CLASSIC; 'NO ONE IS FLYING THE AIRPLANE.' TIGHTER SOP IN REGARDS TO FLT DIRECTOR SWITCHES MAY HAVE PREVENTED THIS. ALSO THE CREW SHOULD MAKE SURE SOMEONE WAS 'FLYING THE PLANE' AT ALL TIMES.