What constitutes 'severe duty' to a spark plug - re 98 Ford Club Wagon 5.4L

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?
I would especially appreciate any shared experiences on this matter.
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Severe service for a spark plug could be lots of high load operation (hauling heavy loads, towing, etc.), or lots of short trip driving that includes a lot of idling. The mileage guideline are based on an assumed number of engine hours per mile. Anything that requires more engine hours per mile driven impacts this.
If you have a stalling problem, I'd suspect the IAC (Idle Air Control). Depositions can build up on these and affect there ability to stabilize the idle. They are easily removed and replaced. Cleaning may be effective (or may not). A new one is less than $100 at the dealer.
Ed
Jerry wrote:

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