We have had this problem on two of our customer sites. In both cases, the DTM would lose sync with the PRI/T-1 circuit for a second or two, and then sync back up. The free run error pointed to the carrier, who (of course) denied that THEY had a problem. Once we had a vendor meet, and they saw the error, and did a little more in-depth checking their end, the problem went away.
Good luck!
E.