Hello Thomas and :welcome: to iboats.
When you purchase a secondhand boat (or car) you should assume that it is overdue for a service. The very first thing you want to do is a FULL service on the engine and the drive. That would include, replacing the water pump impeller, changing the drive oil, servicing and tuning the engine. As well as all the other things that need to be checked on a boat and its trailer. A full list of items to be serviced/checked can be found in the service manuals...
As far as the gear shifting problems are concerned, have a read through the various 'stickies' on the forum and get yourself acquainted with exactly how the shift system works. Once you understand it, then you'll have a chance of troubleshooting it.
Without further exploration, it does sound like an amount of 'play' has developed in the system, and the cable may be out of adjustment. There are procedures for checking the play, and for adjusting the cable. These are outlined quite clearly in the factory workshop manuals (avoid the aftermarket manuals as they have proven errors and omissions). Also in various videos, some of which can be found in the stickies.
Get a hold of the manuals, either by buying them online, or reading through them at
http://www.boatinfo.no/lib/library.html The manuals for your engine and drive are #10 (for the engine) and #6 (for the drive)...
Chris......