Question for DME UPDATING:

Every flight during first 150 nm of Flight Plan, INS gradually starts to loose signals.It goes to 9 from 0. I establish  "DME Position Updating".
Signals slowly goes back to "0". Then during the next 150nm, it starts all over again, signal slowly goes back to 9.

Is this a normal INS procedure ? (I dont think so.Otherwise just fly VOR to VOR)
Do I need to do DME Updating always? >:(

Thx