[fireftr18]

Welcome aboard Bob.

I'm assuming you're using the version of RSRD for RFB. If you didn't, then it will cause problems.
The problem may also be with how you save. There is a known and unresolvable bug with saves in sh4. If you continually overwrite saves, you will eventually cause corrupted save files. What you described is what happens.
When you save, you need to save in a new file. I make a master save in port (ex. fireftr18). Then while on patrol, I add a number to it (ex. fireftr18 1). As I build up save files, I'll delete the earlier ones to keep the total number of files down.
In the past, people also came up with a set of rules to make sure you have the cleanest save possible. When saving while on patrol, save during the day, calm seas, no storms, while surfaced, and nothing within sensor range of your boat. With newer computers, the problems have decreased, but there may still be problems. I haven't had any problems breaking the rules while saving.
And don't "alt-tab" out of sh4. It also tends to cause problems.