1997 Chevy Silverado rough runninng at times

My truck runs good when I first start it, then as it warms it misses or seems to run on 7 cylinders. As I drive it it will go back to running fine if I seen to hold RPM's around 2500 steady. It always will go back to missing at some point. I do not drive the truck everyday. It could use a tune up, I'm not sure if simple tune-up things are the problem, it has had tune up parts not too many miles ago. I have had the truck from 50000 miles on. I have done some work to the motor and wiring over the years I have owned it. Thanks for any info or direction to go it thanks scott