After stopping at a traffic light my '94 Miata often - but not always - emits a massive blue cloud of smoke, which smells like burned oil.
Compression is good, so don't think it's rings. It does not smoke at all when cruising at normal speeds.

Ideas?

 

You may want to ask at http://www.miata.net/
They have a lot of people and as you can see most posts in this section do not get any responses. Plus it is all Miata owners.

Smoke from the tail pipe is usually an indication that you are burning oil from somewhere, which i am sure you already know by the smell.
Have you been checking the oil levels consistently too see if there is any noticeable drop in levels?