A high rate of presession in a vacuum driven DI is due to either.
a) Lack of vaccum pressure at the DI. (Check Vaccum Pressure Indicator. Needat least 5". Low pressu suggests a blocked filter, a leak or pump problem. Be aware! some DI'd may have a seperate filter.
Note: The DI is the most sensitive instrument here. Other equipment is more tolerant.
b) Bearing wear (ie USGyro).
I would recommend that you:
a) confirm pressure is good on the indicator (as above).
b) check that the pipe to the DI is not blocked or kinked.
c) If all of the above is ok then remove the DI and send for bench test and repair.
Dave Goodison at 'Aircraft Instruments Ltd' should be able to check this out on the bench for you.
Bear in mind that precession varying with engine rpm change is also indicative of a vacuum regulator fault.