SX-M will not shift into fwd OR reverse after "warming up"

Hi all,
Boat is a 2005 fourwinns with VP 5.0 GXI-E and a SX-M with 1.60 gear. I been having an intermittent shift issue that no-one has been able to solve yet, basically, after I run the boat for a while, and then shift it into neutral, It will not go back into either gear, fwd or rev. Shifter still moves, cable still moves shift lever on the back of the drive etc. The drive was a remanufactured drive, it now has about a 18 months age and 75 hours’ run time on it, and the shop that built it has rebuilt the upper twice now, at their expense…all new cones, cups, gears, shift shoe, etc. So to pick up where I left off on another forum (which has knowledgeable people and has been informative, but have not found the answer yet), I am going to change the control box and shift cable myself, …
So…I am waiting on the shift cable, but after running the boat today, I have convinced myself of 2 things. First, there is no way it could be the shift cable or control box and Second, it has to be the shift cable or control box, it’s the only thing left…So what does it do, when it is first running, it is good to go, shifts smooth and instant. After I have run it for a while, at cruising speed, it will not go back into forward or reverse after taking it out of gear, but sometimes it will go into gear once or twice, but as soon as I turn the steering wheel it will not (never pops out of gear, but once shifted out of gear, will not go back in). Move the drive up and down, side to side, and let it “cool” for 5-10 minutes, and it is all working again… When it is NOT going into gear, the shift cable is still turning the linkage arm on the back of the drive, and it looks like a full throw. New cones, cups, gears, lube…buy Volvo Penta certified mechanic (at their expense). I will change the cable and control box out when the cable gets here Sunday, but I am not holding my breath it will fix it. I just cannot believe that this hasn’t happened to someone else, these SX-Ms have been around for a long time…I would say the lube is overheating, but that doesn't make any sense either. Anybody got any ideas? Thanks in advance for your help.

Saw your posts over on other forum, some of same guys here as well. One question that a few made comments on but wasn’t clear if it was tried - did they/you try VP lube vs merc hi perf or other lube?

Never have experienced it myself But have seen a lot of post talking about this and cone clutch slip. I think on this forum someone had a thread where they had seen this repeatedly working at a VP dealer. Maybe you can find that thread in the search function. Not trying to be flip or give you the same answer but symptoms I recall from this thread seemed similar to what you have expressed

to rule out the shift cable or control box can you get out into open water with a buddy driving, while in gear and idling disconnect the cable in the engine compartment ( I might be smoking some thing here)? Drive around in open water if you need to stop throttle back and turn off engine ? Just a thought to isolate it as I agree with your two theories....