Depth control, or lack thereof, on XVIII
 

Didn't find this anywhere else, but please pardon if redundant...
I've started spending more time in the U-Boats. However, now that I've been "upgraded" to an XVIII, I have problems. If I submerge to, say, 80 ', the boat'll go deeper and then rise back to something near 80' - and then bounce around. Further, when surfacing, the thing "porpoises," popping up to the surface and then submerging again. This happens EVERY time after I submerge for the first time and then attempt an appearance on the surface. It also does this consistently whether in "Career Mode," or using one of "Syxx's" excellent German Missions.
Any ideas?? Doesn't do this with any other boat!

perhaps its the speed?
usually, the faster the boat goes, the more likely it is to bounce all over the place

Finally gave up...
 

...and went back to an earlier mission before I was offered the XVIII. Depth control was so bad - with or without the Walther motor engaged - that game play was neither fun nor realistic. Coming to periscope depth from either the surface or the deep would cause the boat to bob around like a air-filled ball that's been held underwater. It (literally!) plunges back and forth between 14' (periscope depth is 52-55' below keel) and 160' - even in real time and at "quiet speed." After getting shelled heavily by local DD's due to "conning tower exposure," being unable to fight back as I'm on a roller coaster, etc., I finally gave up and went back to "Old Reliable." May not be sexy or especially fast, but at least I'm not HELPING the enemy sink me!