240d eats # 4 glow plug every day

This problem is driving me nuts. Last week the fuse holder on the firewall of the 77 240d had melted and blown the fuse. Put in a new one and all was fine for a day then the #4 plug burned up (one closest to firewall) Went and replaced all four plugs with new bosh (old style loop types) and the #4 plug has burned up at the rate of one a day since. Just plain melted.
What I have done....

Used a GP reamer to break out any old carbon none was found.
Used a very small magnet inside each GP hole to look for old GP parts (none found)
Turned motor over several times before install of new plugs to blow out anything
Checked relay with a test light across plugs and it goes off as soon as car starts or as soon as dash light goes off.
Checked for short in system and power is going across all four plugs

Each time it took out the #4 plug I use a magnet and get all the melted chunks out of the pre chamber. Then crank it over for several seconds to blow anything else out. Never see anything except a mist of fuel.

Replace plug and car is fine, can start and run several times and drive around the block then park for an hour or two, go back out and all is fine. Give it to my Daughter to drive to work and to friends houses and by the time she comes home at night she tells me the GP light is out but the car starts fine, just takes longer.

Sure enough go out and the #4 plug is dead.
She knows the difference between acc and on so I know it's not her letting the car sit and cycle the plugs for extended times. Fuse has not blown since I replaced it, and while I have been driving around I never have noticed the GP relay light comiing on so I don't think the relay is faulty.

Is there anyway the pre-chamber could be bad on this pot and cooking the plug? Car runs fine, no miss, does not use more than a 1/2 qt. of oil in 3000 miles etc.
Should I scrap the whole series plug thing and go with the fast glow upgrade?
Anybody have any ideas?