I had this exact problem on a '94 306 XT. It turned out to be due to
wires shorting out because of wear in their insulation. It's happening
where the wire bundle from the front doors pass into the body. Take a
look to see if there are any breaks in the rubber covering that protects
the wires. Mine was split on both fron doors. Moving the bundle of wires
around seemed to give temporary respite.

I thought that it had been fixed in later years.

In the end, the cure was replacement of the wiring from both front doors
to the body. Not cheap.

Raymond