Engine Woes

[whale]

Well, after going over the fuel lines, filters, bulbs, vent etc everything had run fine for a couple of trips but then yesterday I had the problem again. Engine starts and runs fine for about 15-20 minutes at cruising speed then bogged down and got worse and worse. We limped to the fishing grounds and drifted a while. After that it ran fine again on the short repositioning moves between drifts and ran fine all the way home. I am starting to think it may be an electrical issue????