Megastream leased lines

Hi,
Our corporote network is linked via 2MB leased line, with Cisco routers at each end. Cisco 1720's. Link is ok, but occassionaly drops for 10-20 second intervals - I have been advised this is clocking timing issues and that 1 router should be set as Master, the other as Slave - I don't have a lot of cisco experience, but appreciate if anyone has any idea how this can be done - if experts required, how long should this job take ?
cheers
Steven.