The Bristol 170 was prepared for takeoff from Guernsey runway 28. The first officer was flying the aircraft from the left-hand seat. The throttles were opened slowly to full power, to combat the effect of a 17 knots crosswind component. As the plane reached a speed of about 50 knots along the runway, the port engine rpm began to fluctuate and, at a speed of about 80 knots, the takeoff attempt was abandoned. When it became apparent to the crew that the aircraft would overshoot the end of the runway, it was steered to the left to avoid obstructions. The aircraft became airborne for a short distance and ran through the airport boundary fence and across a public highway, coming to rest 1/4 mile from the end of the runway. PROBABLE CAUSE: “The captain abandoned the takeoff due to a malfunction of the port power unit but was unable to bring the aircraft to a stop on the runway remaining.”