Need some help with my Miranda cameras

A few days ago I've got two Miranda cams (Sensorex and Sensomat) and btw both of them have the same issue. Shutter speeds from 1/30 to B didn't work correctly. Shutter closes too fast (with the same speed on every position), and the mirror sometimes gets stuck in the upper position closing the pentaprism. So, what's a reason of that problem? May it be an old oil only, or something more serious?