2001 E150 headlight short

When headlights are turned on, they stay lit for about 5-8 seconds, then begin flickering on/off every 3-5 seconds. I have replaced both left and right fuses under dash. I noticed that when just the left fuse is in the left headlight will remain lit, same with right, just not simultaneously. Would this be some type of relay problem ? Thanks for your help:)

try a new relay