For some time now my Motion4 have been acting out, when i render. I command+R -and see in the preview window it's being rendered, frame by frame, but when it reaches 100% rendered, and the window dissapears, the timeline render-field (The green bar) goes back to zero again!
I've had a somewhat similar problem before. Have you tried clearing your RAM preview? I've found that if I do that periodically, it tends to make the RAM preview work better... It's under Mark > RAM Preview > clear RAM preview.