FLC OF A B737 AT CRUISE HAS A PROB WITH AN 'L' VOR NOT GIVING THEM ACCURATE INDICATIONS ON THE CLRED RTE. ATC ADVISES THEY ARE 6 DME OFF COURSE.

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WE WERE CLRED DIRECT ESL VOR. WE HAD TO ASK CTR FOR A HDG BECAUSE THE VOR IS SO WEAK. AFTER ESL VOR; WE WERE CLRED V377 TO HGR VOR AND ON TO HAR VOR. AROUND TOMAC INTXN WE LOST DME OFF ESL VOR AND HGR VOR NEEDLE WAS SWINGING BACK AND FORTH. CTR TOLD US WE WERE 6 DME NW OF HGR VOR. WE TOLD HIM THE PROBS WE WERE HAVING; IDENTING TOMAC INTXN TO MAKE THE TURN ON THE AIRWAY. I DON'T THINK CTR SHOULD ISSUE A DIRECT TO A VOR THAT IS AN 'L' CLASS VOR; WHEN YOU ARE TOO FAR AWAY TO RECEIVE THE SIGNAL. BOTH OUR VOR NEEDLES WERE UNRELIABLE ON HGR VOR SO I DON'T BELIEVE IT WAS OUR EQUIP. IF THEY (ATC) WANT US TO NAV; WE NEED A RELIABLE STATION TO DO IT PROPERLY. THERE WAS NO INCIDENT INVOLVED. SUPPLEMENTAL INFO FROM ACN 429887: FACTORS AFFECTING OUR NAV INCLUDE: 1) DISTANCES AND TERRAIN INFLUENCES OF THE USE OF LOW PWR VORS; INCLUDING ALTS; FOR FLT PLANNING PURPOSES. 2) STRONG INCLINATION OF INTERFERENCE OF AN ONBOARD ELECTRONIC DEVICE BEING USED BY A PAX (PHONE; LAPTOP COMPUTER; VIDEO GAMES). I HAVE SEEN THIS SAME TYPE OF INTERFERENCE BEFORE WITH ELECTRONIC DEVICES.