On my NA there was a problem with the small wiring harness on the turn signal housing itself - swapping bulbs from one side to the other did nothing, but temporarily swapping the entire turn signal housings moved the problem from one side to the other. I had a spare (cracked) turn signal - took the harness off that and the problem was fixed.
I can see how this could have manifested itself as 'intermittent', but I was lucky - there was nothing obviously wrong with the wiring harness to look at, bulb holders looked good etc.