From memory, I think the bad earth on the front ones is on the money.
I think the bulb holder earth return connection is via that little flat strap inside the lamp fitting. In turn, this strap is connected to one of the mounting studs that pokes through the apron. In an imperfect world, there is a chance that the stud passes perfectly through the hole, without ever touching the sides of the hole, and thus, the only way the earth return is made, is iF the speed nut you put on the stud, actually grinds enough paint off the inside of the apron, as you tighten it up.
In a recently painted/restored car, the above is probably likely not to occur.
Thus you get the wacky feedback issues like you have?
I tried those little "star" washer thingoes, but that was not enough...in the end I ran an earth wire from the stud and connected it to somewhere where it was not seen via a self tapper ( that should cut through all the paint etc).
Hope that all makes sense.
To check this is the problem before launching into the above solution, just get a little alligator clip lead, and go from the earth strap inside the lamp fitting, to a good earth on the body / or the battery -ve. If the poor earth IS the problem, it will instantly dissapear.
p.s. also double check you have the correct globe and that it is inserted the correct way.