While up on plane on a 280 with twin 4.3 L V-6’s/Alpha drives the port side engine suddenly began to over rev. The boat lost speed and came of plane. Put the engines in neutral and trimmed the drives up and then back down to clear any debris that may have gotten picked up then put the engines back in gear and attempted to get back on plane. The port engine again over-revved and the prop had no bite. The starboard engine ran normally. Stopped the boat and trimmed and lowered the drives again. This time it got up on plane but after running at 3500 rpm for about 20 minutes the same thing happened to the starboard engine. Repeated the same exercise successfully. There were one to one and a half foot waves and some debris in the water. Had a full load on the boat since then withou any problem. Any ideas? What should we check?