Sometimes sounds like it's spitting out a hairball

As with all the best problems this one is defying reproduction. Sometimes I will start my 97 Ram2500 truck it coughs and fusses and shuts down. I restart and give it a lot of throttle, acts like it's hacking out a hairball, and eventually it will start and blow out a large cloud of gray/black exhaust.

Then runs like there was never a problem until the next time.

Only things that seem to have any relation is sometimes parking on a slope, doesn't matter up or down. Sometimes having sat for awhile. More often when it's cold.

By the way, never happens in the presence of any of my mechanically gifted friends.