Smoking left bank - SOMETIMES

In the past year I have witnessed oil smoke expelled from the left side exhaust exactly two times. Saturday was the second time and the first was about a year ago. It's not just a bit of smoke, it's substantial. Both times I witnessed the smoke it was as I pulled into the garage, I could smell it before seeing it. I don't think it's happening at other times as I've always got the top off and I'd smell it. The engine is a TPI 350 out of an 87 Vette with a Chevy experimental cam with lots of lift. Everything else is pretty stock, except for headers. I had problems a few years ago with constant mild smoke. That was found to be the result of the high lift cam (.603"). The rockers were hitting and destroying the umbrella seals but a bit of machine work resolved that problem. I am using a small amount of oil now but the oil stays clean (maybe 1/2 quart every couple thousand miles). I haven't pulled plugs for examination but the smoke occurs so seldom I wouldn't expect they will tell me much. I started the car up this morning and the exhaust is invisible - just like the first time it happened. Both times I witnessed smoke the engine was running just a bit over the 180° but was not puking. I'm baffled - maybe that doesn't take too much. Any ideas are welcome.