Volvo Penta 150S Saildrive Gear Problems

[SV Sapphire]

We have a VP 150S with only 29 hours on the clock and it has a really annoying habit of not going into gear as soon as we leave the harbour. Selecting forward or reverse in the harbour is absolutely no problem but it seems as soon as the boat experiences any chop outside the harbour, we can't select a gear without rocking the throttle lever back and forth and practically slamming it into gear. I've check the gear arm and cable retainer and tweaked the morse cable but no joy. I'm totally frustrated it. Does anyone have any ideas?

Thanks in advance from a skipper whose boat doesn't want to leave the harbour