Create a tool for improving writing flow

I'm having a continual problem writing that I assume must be common but I haven't seen often described. I suppose it's a version of writer's block, but it's more specific and seems like the kind of thing that people have probably already expressed and troubleshooted for already. I'm hoping to find some suggestions for moving beyond this. I know what I want to write, I know the scenes and the plot and what seem like all the necessary aspects of the story. I feel excited to sit down and write, motivated to get on with it, and optimistic about the direction the work can take. But when the words start coming out, things quickly grind to a halt. Nothing reads right, thoughts get immediately disorganized, and the ideas become seemingly impossible to express. The amount of words per minute immediately plummets. Part of the problem may be that my current project (manuscript?) has been largely outlined and organized, and much of the challenge is getting one section to properly connect with the next. At least, this is a dilemma I find myself up against frequently and it wasn't something I counted on. But it presents itself within specific chapters and scenes as well, an inability to properly lay down the words and moments that create a natural flow. It feels like I ought to be able to lock in and write stream-of-consciousness given the level of interest I have in my story and detail in the mind's eye. It's all just sitting there, but the process refuses to allow me to efficiently arrange the right words in sequence fast enough to create any kind of "flow". This is the best I can explain it. Pretty frustrating. Perhaps it's just generic writer's block but I'm not a good enough or experienced enough writer to have ever had a proper flow in place to "block". Is this a thing?