Before you send it off for cleaning/repairing, sit down with it while you watch TV or something and just repeatedly cock and release the shutter. I've had shutters of that general type (I believe including a Mamiya C-series lens) that behaved as you describe, but just running them through a few dozen cycles got them working much better. I speculate that the lens had sat in a closet for long enough that the lubrication got a bit gummy, but working the mechanism a few times smoothed it out.