Keep an eye on your coolant levels. If it drops, either gradually or intermittently for no apparent reason, take it in and have a compression test done.
Sounds like a blown head gasket to this old mechanic...
The reason you get WHITE smoke is you've got antifreeze seeping into one or more cylinders. The reason it's intermittent is that it will vary depending on how the engine was used; wether or not it was stressed hard during its last drive, and so on.
Not the best of news, but nothing's busted, and it can usually be found with a compression check. But you shouldn't let it go on; it's not good for the head to have that hot spot where it doesn't belong. Repair cost varies wildly from vehicle to vehicle depending on what has to come off to replace the set of gaskets on that side of the engine, and how easy it is to get the head off/on.
Hope this helps.
