You may want to check the circuit ground, if you haven't already.

 

Check the grounds first , but also check dual filament bulbs cross connecting at the filaments .

This occures when a bulb is fitted with two filiments at a horizontal position.
One filiment will open and fall down on the second and cause a short between the two. This condition will keep you scratching your head for weeks .!

 

Simple test of the bulbs would be to switch the left and right bulb out with each other. If the right acts OK, but the left gets squirrley the it is the bulb.