After posting this thread a couple of synapses fired off
and I remembered that the "crash dive" function may hurt your depth keeping abilities. I had commanded a crash dive AND changed the depth from 70m to 200m. Not sure if the crash dive itself or the depth change from default 70m caused the depth holding problems. Either way, I replayed the convoy attack and did not use the crash dive function. Instead, I commanded Flank speed and set depth at 200m...and had no problem maintaining 200m at 2 knots and silent running. I also noticed that the escorts had a much harder time "seeing" me at 2 knots compared to 2 knots after a crash dive initiation? Not sure why but I'll not be using the crash dive function again!