Hi Peter
Thanks for the tip on the High DPI behavior. This made life significantly better (although didn't fix the issue). I've read online that lots of games have exhibited odd behavior due to this setting. Also, I have a widescreen monitor but am not playing in widescreen. Also noticed that Synchronization" (or V-Sync I assume?) wasn't enabled, so enabled that which also improved life considerably, but also did not solve the issue.
I did some digging around in Event Viewer to see what was causing the crash, and found Kernell.dll was the module at fault. Previous knowledge and some further Googling confirmed this to be a memory related issue. Other player have found that SH3 is limited to using 2GB of RAM at any single time. This explains why when I was introducing new additions to my sub, this was hitting the game's limit of RAM it was allowed to use.
More Googling discovered "Large Address Aware". A free piece of software that modifies the .exe file to use more than 2GB of RAM. I've used this method in the past for older games like Sid Meier's Railroads, to great effect. The author of the software has confirmed his tool works with SH4!
I can confirm that using Large Address Aware has solved the issue! I made a backup of the unchanged .exe first. I have successfully made many further additions to my sub and play a further 5 patrols with no issues. The game is also significantly more stable now, no more random crashes etc (clearly caused by the game hitting its RAM allocation).
Large Address Aware is available here:
https://www.techpowerup.com/forums/t...-aware.112556/
I can make a handy dandy guide on how I did it if you require
Many Thanks for you help and have a a great New Year
-Alex