Hello Alfapiomega.
You mention you're using Windows 10, with a Steam download...having both the original SH4 (which by the way, Steam provides version 1.3....not the latest original version which was v1.4) and the U-Boat Missions Add-on.
May I suggest you follow this thread...."
Windows10; Large Address Aware; SH4 Install" for properly installing Windows 10; How to install the game on a Windows 10 computer; Adding the very important Large Address Aware to your game to use the memory you no doubt have with your newer computer.
It sounds like your issue has to do with the game faltering as you play. That to me can easily be due to the game not utilizing the memory your system has. FOTRS adds a lot of 3D model content....which requires memory to render. Forcing the game to use the memory you have (which is what LAA does) may correct your issue.......besides making sure Windows 10 is setup properly to run as expected.