Citroen Xsara Picasso headlight bulbs keep blowing

Have had this problem now for about 12 months, the dipped filament on the headlights keep blowing. The problem is not regular, it’s intermittent sometimes it’s the left one sometimes the right one, there is never a set time when they blow but it’s almost every 2 to 3 months. It’s only ever the dipped filament that goes? Never the high beam. I have fitted cheap bulbs and they would last weeks, the Philips ones and as is now the Osram ones are the ones that seem to last a few months. I have checked all the fuses, cleaned the contacts wd40ed every wire I could and the whole fuse box but still no change, the local dealer didn’t have a clue? No surprise there. Please help will soon need a second mortgage to keep paying for the bulbs.