Mamiya 645E's shutter problem

I just got a Mamiya 645E from my local camera store. I'm currently testing it with a roll of Fomapan 100. It seems to work well except when it doesn't. There is a problem with the shutter. It doesn't seem to be an issue usually. The shutter works fine, but sometimes it stays open, and I have to close it by releasing the shutter button and pushing it again. Any ideas on how I can overcome this, most likely and hopefully, design issue?