Boat engine playing up?

hi there , my boat engine is a lister air cooled and it always loses power in the first 20 minutes or so after start up, i have changed filters , checked all connections, it sounds like air in the system fuel starvation, but once it warms up its fine, why does it only lose revs on the way out to the fishing grounds but never on the way back, any ideas why it does this?? many thanks