CAPT OF A LEAR JET 35 DEVIATED FROM ASSIGNED ALT AND TRACK AFTER ENCOUNTERING TSTMS CAUSING APPREHENSION BY THE RPTING FO AND ATC INTERVENTION TO MOVE OTHER TFC IN CASE THE DEVS CONTINUED.

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WE WERE GIVEN A CLRNC TO LAND TOWARD ILM ARPT WITH THE CLRNC TO DEVIATE AS NEEDED AROUND WX. RADAR SHOWED ECHOES OF MEDIUM INTENSITY ALONG OUR INTENDED FLT PATH; SO WE STEERED TOWARD THE AREAS THAT SEEMED TO BE THE LIGHTEST IN INTENSITY. THE CAPT WAS FLYING FROM THE L SEAT AS WE ENTERED THE CLOUDS AND BEGAN TO ENCOUNTER MODERATE TO OCCASIONAL SEVERE TURB. HE THEN SAID 'WE HAVE TO GET OUT OF HERE' AND BEGAN A 45 DEG BANKING TURN TO THE L. IN THE TURN HE LOST APPROX 600 FT. WE THEN BROKE OUT OF THE WX AND ADVISED THE CTLR THAT WE WANTED TO CHANGE OUR DEST TO WV. WE THEN CTRED THE CLOUDS AGAIN AND WERE CLRED TO 8000 FT. THE CAPT THEN BEGAN TO DEMONSTRATE POOR ALT CTL AS LIGHT TO MODERATE TURB BEGAN. I CALLED OUT HIS DEVS AND ASKED HIM WHAT HE WAS DOING. THE CTLR ALSO CALLED AND ASKED WHAT ALT WE INTENDED TO MAINTAIN AS WELL. THE CTLR BECAME EXCITED AS HE HAD TO GIVE RAPID INSTRUCTIONS TO ANOTHER ACFT DUE TO A POSSIBLE LOSS OF SEPARATION. I WAS ASSERTIVE IN MY REPONSE TO THE CAPT'S UNSATISFACTORY ACFT CTL. I DIRECTED HIM SEVERAL TIMES TO 'GET ON HIS ALT.' I FEEL THE CAPT WAS NOT VERY CURRENT AND WAS SOMEWHAT RATTLED BY THE PREVIOUS TURB. IF THE CAPT CONTINUED HIS ALTDEVS; I WAS WAS GOING TO TAKE CTL OF THE ACFT AND TALK ABOUT IT LATER ON THE GND.