When I restored my R&B, I bought complete DE flipper rebuild kits that included the spring washers. I did everything exactly as specified by the manual and they have been perfect ever since. Although in OP's case, it sounds like shoddy workmanship on the coil stops like others have said. Might Should just try a different parts dealer. I mostly get my hardware from Marco.
Here's what they have for DE coil stops:
http://www.marcospecialties.com/pinball-parts/515-5346-00
And the correct spring washers: