Bubbling back into overflow coolant tank ?

I have a 89' S-10 with a 4.3 v-6 , after you shut it off, the coolant in the overflow tank starts bubbling . This goes on for about 4-5 min. and then stops . It also sounds like the engine 1/2 misses every once inawhile during idle. Why I say 1/2 miss , is that it doesn't miss as to where the engine shakes alot as in a definate spark miss. I hope I made this clearer than mud. Anyone have any ideas as to what is wrong ? I just replaced the sparkplugs tonight as it had been awhile and they actually looked pretty good. Thanks, Iowa883