#5 cylinder not firing

We have a 1979 Cessna 182RG with 1350 hours since overhaul. 6 weeks ago,
we began to notice the engine to be rough on the right magneto during
run-up. Over two weeks, it became increasingly difficult to burn it off by
leaning during the run-up, until one day when it became impossible. We
noticed on the engine monitor that the #5 cylinder head temp was cold during
the run-up. Our mechanic looked at it, and yep, it was a fouled spark plug
which was replaced. Now, about 15 hours later (and an annual), we have the
same problem with the #5 cylinder. It's going back to the shop today. Any
ideas? Why the same cylinder? What should we be looking for?