I have recently had this kind of experience with my PSTN, i am Network Manager in a Fixed wireless Access S-CDMA enviroment.
I dont know how you interconnect with your PSTN, but, based on my experience if it could be of any assistance, we have a fiber optic cable with Multipexers(MUX) in-between my network and the PSTN.Ensure your E1/T1 interface connecting to your PSTN on the MUX is set to use the PSTN clock (remote/line depends on MUX settings),all these clock seetings are at the Physical Layer and not the Layer 2 Clock settings on your Switch and the PSTN.
Hope this will be of any help.