I am not really familiar with these drives, but it sounds as if the hydraulic pump is somehow loosing its "prime". That is, liquid somehow leaves the pump and it is unable to start pumping new fluid right away.
I have not looked at the design, but usually these pumps are submerged in the fluid to prevent this type "leak down". The only thing to do is fluid and filter change.... after a thorough washing to make sure no grit gets into the system.... which you have already done. So my only question is about the AGE of the fluid and filter you removed.
Unless this is a known issue with your model, the issue is tiny and therefore (in my estimation) the problem will be hard to find, and thus expensive to fix. I suggest you drive it through the summer, trying to detect if the issue is getting any worse. Here in the middle of winter, it could simply be a viscous fluid flow issue. If it's doing the same thing, or getting worse, when it's 75F and above... then you have a real issue.
PS. Posting your location would help. Knowing if it's -20F or +85F where you are would be a clue.