I've had quite similar problems with my 132c on an AFD camera. Not sure about the construction of Sinar adapters, but perhaps the cause is the same as mine. Take a look at the back-to-camera contacts on the adapter and compare them to the contacts on a filmback. You might notice, that the contacts on the DB are slightly shorter. If this is true, your contacts may be worn out (or, more precisely, the tiny springs, that push the contacts against each over). You can try inserting small pieces of metal (eg. foil) between them to ensure good contact and if the problem is gone — this is your cause. In this case you can either get a new adapter, or try to fix the old one somehow (not sure if this is possible).