What constitutes 'severe duty' to a spark plug?

The service guide for my '98 Ford Club Wagon says that my spark plugs should be replaced at 100,000 miles. With 75,000 miles on the van now, I've got mysterious stalling problems when
approaching an intersection; i.e, when decelererating.
I strongly suspect the engine is running lean. The leanness is not overwhelming, however. There is some pinging, while on the freeway, especially during the warmer times of the day, that was never evident before. The timing is OK, according to the OBDII break out.
Nevertheless, it would probably be a good idea to change the spark plugs. After all, a few misfires, especially as low idle, might cause an engine to stall. So I looked at my service guide, and sure enough, it says I'm early if I only have 75K miles. But wait, there is a "severe duty schedule" that says the plugs should be changed at 60,000 miles.
This van gets a great deal of work on short-haul trips. I have heard that this is hard on the engine lubricant. Would it wear spark plugs prematurely? Why? Because they're "cold" much of the time? Does "cold combustion" age spark plugs at an accelerated rate?
Thanks for any insight you might offer on this metallurgical subject, of which I know nothing.
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Hi Jerry,     The spark plugs may last 100k moles but I would think they would need to have the gap set several times in that interval. The metal does erode away and the gap will get wider over time. A wide gap can cause pinging and stalling.
-jim
Jerry wrote:

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