1994 Idle Issue

Took the truck for a spin today and when i come up to a spot and if i let the truck just idle as i have my foot on the break it seems to idle below 1thousand rpm's and acts like it wasn't to die. I asked around and was told that it's normal but never has my truck done this befor. And, some times if it's in park idleing it will idle normal then all of a sudden get really low and that will continue on through the idleing , would go good then idle rough again, what could be the problem? Changed the sparkplugs/wires/cap/rotor not even 3months ago and the truck just has been sitting, but i start it just about every other day and let it run for about 15min or so.

Any and all help is greatly appreciated.