I have a '99 135 Optimax that has been in and out of the shop several times in the pass few weeks. I have faith in the local marine shop but they have checked everything they can think of. They have been on the phone with a Merc tech several times and he is scratchin his head too!! When they hook it up to the computer it shows no faults. Mercury says they will send a factory tech to look at it but it will be at least 3 weeks before he gets here.
When I first leave the ramp in the morning it gets up on plane just fine reguardless of the load. After that it starts to struggle to get up on plane and sometimes won't get over 2500rpm. To me it seems like it could be temperature related since it runs fine at the beginning of the day. Once it gets up on plane it will run fine all day at 5k rpm.
I'm hoping that someone out there has had the same exact problem and can tell me what the problem is. As I said, the local marine has tried everything they can think of plus what ever Merc has told them to try over the phone.

I had a Merc EFI do almost the same thing and it was the temp sensor in the heads. I don't have any experience with Opti pops. Mine was running on default and the computer was putting too much fuel. That should be easily found though. If i had to take a wild [censored] guess since the computer isn't showing anything..I would say possibly the tuner housing is burned up.

Are you getting any warning sounds?

DO you have Smartcraft instruments so you can see what the motor temp is? If the motor runs too cold it can foul spark plugs at idle and then when they clean off the motor will run fine again.

You may have a fuel pump that after it runs for some time it heats up and starts binding up inside and is unable to produce needed fuel pressure. When at the shop they are checking it cold and fuel pump works fine. Not a common problem but has caused myself a diag. nightmare or two.

Tommy may be on to something if the T stats are stuck open or ir the head sensors are not failed open or shorted but just reading wrong it not set a code but run cold

Also are you sure the horn is working? If it has not been checked, you need to. Also, one of the fuel pumps may be getting hot and shutting down.