This is another example of the 'Crash Dive Blues'.
It is a bug in the original game, but it is worse with NYGM because the U-boat is trimmed to sink at slow speed.
THe problem has been described many times in the past, for example:
http://www.subsim.com/radioroom/show...6&postcount=50
Follow the solution given in the link, and you will have no more problems.
Stiebler.