THE ACFT TOOK OFF WITH 4 OCCUPANTS & AN ESTIMATED 596 LBS OF CARGO ON BOARD. AFTER IT TOOK OFF, THE ATC CONTROLLER NOTED THAT IT WAS CLIMBING SLOWER THAN EXPECTED. AFTER CLIMBING TO ABOUT 4200 FT MSL, THE AIRCREW REPORTED A PROBLEM WITH THE LEFT ENG & SAID THEY DID NOT BELIEVE THEY COULD MAKE IT BACK TO THE DEPARTURE ARPT. THEY EXPRESSED A DESIRE TO CONTINUE TOWARD THE SOUTHWEST. ATC PROCEEDED TO VECTOR THEM TO THE WESTON ARPT, BUT INFORMED THEM THAT IT HAD NO INSTRUMENT APCH. AFTER THE REPORTED ENG MALFUNCTION, ATC PERSONNEL NOTED THAT THE ACFT CONTINUED IN A GRADUAL LEFT TURN. ABOUT 8 MIN AFTER TAKEOFF, RADIO CONTACT WAS LOST. A WITNESS ON THE GROUND SAW THE ACFT COME OVER A HILL WITH THE SOUND OF AN ENG AT HI POWER. AT ABOUT THAT TIME, THE ACFT COLLIDED WITH A POWER LINE CABLE, THEN CRASHEDINTO 2 TREES, IMPACTED THE GROUND & BURNED. AN EXAM OF THE WRECKAGE REVEALED NO SIGNIFICANT PREIMPACT FAILURES. THE ACFTWAS ESTIMATED TO BE APRX 400 LBS OVER ITS MAX CERTIFICATED GROSS WT.