Note: I have used the code from GitHub - Pantong51/AdvancedMovementComponent: Jetpack added to the CharacterMovementComponent in UE4: Network Ready, Supports Rollbacks, Pretty basic for your own needs
which seems to get a lot of its design from the tutorial A new, community-hosted Unreal Engine Wiki - Announcements - Unreal Engine Forums
I’ve probably look at that tutorial at least once a month for half a year now, and I think I have a better understanding now of it all. 
The problem I’m seeing still seems to happen because I think I have a variable acceleration rate. Now if it was happening extremely often I’d suspect it would have been a similar symptom to changing MaxWalkSpeed or something every tick. But everything seems perfectly fine up until the point where you get close to the maximum speed. Thats when the reset of positions happen and the sudden loss of speed.
I also tried today a maximum speed of 500, and the issue did NOT seem to happen. But DOES happen when a max speed of 600 is set.
Yet still I can set the maximum speed to 2000, and just have a very low acceleration up to maximum speed and I dont seem to get the resets?