Hi Albacore and welcome to JA
The next thing I would try would be the engine thermostat to see if that is the problem as the do get weak over time and should be replaced about every 2000 hours. The other thing that I would ask would be if the aftermarket gauge is electric or manual. If it is manual then they usually don't give much problem and are fairly accurate. If it is electrical make sure that the gauge and sending unit match and that you have good voltage at the gauge. 0.5 volt drop at the gauge will cause the gauge to read about ten degrees to hot.
Marine Mechanic
marine diesel technician for over 20 years.