I did the same thing last night.. had like 10 ft swells.. stayed surfaced almost to 1500m from the convoy before submerging and taking my shots. The escorts were so confused they ran about 2nm from the convoy searching for me. it was like shooting fish in a barrell. The whole damn convoy went under and I told NO to resume course and speed as the escorts were STILL looking for me a few miles away. *BONG* CTD. The most exciting and productive patrol since (as you said) coming back to SH4 after a long break and all for nothing! GRRR!
For myself I know all drivers are up to date and things are good on the rig, but I am having trouble with the mod I have installed, some little quirks (shiop wont hold speed in TC, ship wont go to periscope until AFTER doing a crash dive... etc)... what mod (if any) are you running.. and what version of SH4. I am running v1.4 and added RFB for 1.4 and nothing else and since doing that I have had a few CTD, wondering if we are suffering the same issues??