As alluded to above before you do anything I would check the radius of coverage that the beacon you use has. Then check for weather, coasts, large metal buildings, etc etc.
Do you use it IMC? If not I wouldn't worry about it. If so and all the above don't give any reason for doubt then yeah have it checked out. But i wouldn't give the shop a blank cheque, especially as you have a GPS.
I regularly fly outbound on an NDB to turn for a Loc/dme. With the localiser set up, and the GPS I can see I am flying a direct straight line outbound. If I were to attempt to keep the ADF on the exact course I would be weaving about the sky all the way!