Merc 25 multiple problems

I have a Mercury 25 XD SN 6453940 that has given me 10 years of service fairly reliably. I recently had to replace the flywheel after the magnets shattered causing the engine to run on one cylinder. I would like to know what year the engine was made. A continuing problem is erratic running at low or idel after a high speed run of say 5 minutes or more. The engine, which previous to the run was idling smoothly, now requires advancing the throttle to keep running. It appears to be related to heat in that after 5 to 10 minutes of this behavior the idle seems to improve. I have rebuilt the carb and replace the gas, gas tank and line to remove the issue of contamination. I can find no thermosat on this engine. The teltale water discharge appears to be ok both at idle and at speed. I am also experiencing some shift issues. It appears that the adjustment may have slipped. Forward is fine. Reverse is partial. How to adjust? I have heard the lower unit repair runs in the hundreds. Is the engine over the hill?