Directional Flashers & Bulbs

Anybody got experience getting their directional flashers reliable for inspection? I've got a weird thing happening on my 70 TR6. Flasher works on one side, but lights burn constant on the other. I swapped out the flasher (three times over the last 2 years) with varying results. Just recently after looking for grounds and testing the circuit resistance (similar ohms on both sides), I tried swapping the rear directional bulbs, and voila! The flashing side moved to the other side! I bought 2 new bulbs, tried them and neither side will flash with a new bulb in it! I took the "old bulb that flashes", put it back in each side and repeated the test, same result. So it appears I have one old bulb that passes enough current to make the flasher actuate, but can't seem to buy a new bulb that will. (Hazard flasher works just fine on all four corners, regardless of bulbs.)

A few questions for the experienced "Lucas" experts out there:
-Do I need a "special bulb" for this? (New bulbs are Sylvania "long life" type, from local parts store) Will TRF sell me something different than the local parts store bulbs?
-Should I give up on the "original" Lucas flashers from TRF and retrofit a "more forgiving flasher to get through inspection?
-Am I crazy and really dealing with the usual "loose wire" problems and the "magic flashing bulb" is just a coincidence?

Thanks for any help.