A few months ago I put a salvage '92 1.0 litre in my '95 Metro Hatch.
The compression checked out great and I changed the oil & filter to Mobil One synth 5w30.
The car runs fine, with no smoking or noticeable oil consumption 99.9% of the time.

But maybe once every 100 to 400 miles, usually on the highway, it will belch out huge billows of really thick gray smoke that quits after I put the 5 speed in nuetral and let it idle for a few seconds while coasting.
The performance doesn't seem to change during these spells, even though the smoke literally hides all the traffic in my
rear-view.

I have used 4 pints of Seafoam in the 3,000 miles since the engine was installed, putting half in the gas tank and half in
the oil. I've been thinking a ring must be getting stuck but it should have loosened up by now.

Has anyone run into this before? I reeeally don't want to tear it down. I sure would appreciate some help! Thanks!

Probably is a stuck ring. But if it's continuing, there's probably carbon buildup everywhere. Probably needs a ring and hone job.