1996 Nissan Truck Mysterious problem (to me, anyways)

Hello,

My 13 year old truck normally works great: however, if I don't drive it for a week or so (especially in rainy weather), the engine starts to bog down when I put it in gear. And it is only in the mid rpm/power range: it idles fine. Afetr a couple of miles, it works great again. I hate to bring it to the dealer when it is working fine, yet I can not predict whenit will act up again.

Many Thanks,
John