What could be happening at idle

[E'sStrokr01]

I've just about eliminated all problems with the truck running very rough lately. One of the things I've noticed, is that now that I've treated the fuel and tank with PowerService Diesel Kleen conditioner, the truck has run very well and then a couple of days ago, I left the truck running for about 10 minutes while I went into a store, then I came back out, drove for about a block, then stopped at a redlight, then the truck ran sluggish and wouldn't start for about half an hour. Then I got the truck started again, drove for about a mile, and then everything cleared up again, and ran fine.

The truck was rough before the fuel conditioner, and did this several times after being left at idle for a few minutes at a time.

My question is, what could be happen at an extended idle that makes the truck run rough???? Even though very infrequently??? Have I sent the remaining trash in the fuel into the fuel filter? But then again, why does the truck continue to start afterwards???

FWIW, the temp gauge stays right around a quarter to a third of the way from the C. So, the temp seems to stay cool and is consistent after the truck is first started and warmed up.

Thanks

[Southpaw]

Does it get any better if you change the fuel filter?