BlueStarfish, I would recommend two things? I dive in 50 deg water up to 5 hours per dive and this is what I recommend. Use the DUI 400 gram (400 gm/m2) Thinsulate under garment. This is good for 40s - 50o F temperature. Also I would invest in Argon Inflation to fill the suit instead of your back gas? Argon is the choice of gas to use for long, cold dives as it's thermo coefficient qualities makes it a perfect insulator. Use a Luxfer 14cuft AAL bottle and a Sherwood regulator. Turn the Intermediate pressure down a tad, this helps make it more reliable and better for buoyancy control. If you buy or use another brand of regulator make sure you have an over pressure valve installed on it just incase you loose a high pressure seal, this keeps you from over pressurizing the intermediate pressure and bursting the hose. Ok if your getting cold this could mean that your suit is to tight (to small for you) and not allowing you to inflate it enough to put some space (insulation) between you and the water. Or your not using enough gas to inflate the suit. By the way the reason I use Thinsulate is because if you flood the suit the Thinsulate will still give you insulation warmth. Also I wear thermal underwear made by "Hot Chili", they are for snow skiing but there really thin and do a great job for making the extra layer. Well that's my 2 cents, Hope it helps! Between the 400g suit and the Argon you should stay right toasty!
Cavenutz