450C slow to engage

When I first started it up it would not move when I put it in gear. It did move slightly letting me now it was engaging a little. I rev'd it up and tryed different gears with no luck only slight movement in the different ranges. I killed it and checked to be sure the fluid was still full after the recent change and it is. I did notice that the fluid was very bubbly looking like It'd been shaken up. I started it up again and put it in 2nd low and rev'd it up and waited about 3-4 minutes and it suddenly engaged. No more issues from then on. I pulled the dip stick out and listened for an air sucking noise- there was no noise or pressure there (prior to it engaging). This has happened a few times before. Same pattern. It doesn't do it every time but when it does it leaves me with a sinking feeling it may not go this time. After it engages the first time no issues until I try to start it cold the next time (maybe).
I suspect a sticking relief valve but it runs great after it engages. I would think the relief valve would cause issues while it was running. I would think the pilot needle valve but the manual says it will be slow to engage always which I take to mean not just when it's cold an it is sharp shifting after it engages the first time. I would think the pump is going but I believe these have a flooded suction.
The thing acts like it is losing it's prime after it sits and I can't see how this design with a flooded suction could (or is it not a flooded suction?) Little help or what should I try first? I hate to pull the engine and find out that it is just an o-ring or hole in a pipe that I could have checked.