This may be way too simple but make sure the fluid is at the correct level when warm also. If you have trans fluid there is a considerable difference in hot versus cold levels. You could be on the low end of the "hot full" level which would leave you low when the trans is cold.
To give you an example of how heat effects the level.... I changed the fluid on my first vDrive and filled to the full level with the trans cold (apparently I didn't bother to read the fill instructions). The boat ran great and seemed to operate well for a couple weeks in the spring with light duty. I took a rather long speed run back to the harbor a couple weeks later and was horrified to see a considerable amount trans fluid in the bilge when I cleaned up after I pulled the boat for the day. I was able to track the source of the fluid back to the breather. The next day I got everything back to temp and found that the fluid level was still a bit high although it was near the bottom of the fill marks when cold. This may not be your problem but it would be simple to check and may be a easy remedy.