I had this same exact problem in my '85 Coupe. Unfortunately
(and I HOPE this won't be your problem) it turned out to be a
broken stud on the exhaust port of the #5 cylinder. It was a
B***CH to fix, but then in 5000/100 series there is alot more
room to manuever, unlike the coupe. What made it even worse was
that it was the bottom stud. This was a couple of years ago and
the dealer wanted $800 to fix it, and being a poor college
student, I decided to do it myself. Of course, it was all labor.
I had to remove the intake and exhaust manifolds just to get to
it. Between the Bentley and Haynes manuals, I somehow managed.
The parts amounted to new exhaust and intake gaskets, stud, lock
nuts, and a lot of soap to wash out the mouth......
Let me know if you have any questions