Intermittent Blue Smoke & Oil Consumption

On three occaisions, all when the temperature has been below 15 degrees F, my '98 Jetta produces clouds of blue smoke for several minutes, up to a 1/2 hour on one occaision. Monitoring of the dipstick shows the level dropped about 1/4" after the last episode. This seems to happen about once a month. The car has 43K miles on it. I bought it with 33K on it this past September, so it had mostly been used for short trips. Delvac is pretty expensive to be blowing out the tailpipe - can anyone tell me what's causing this? I was thinking a bad turbo oil seal, but if it was that, wouldn't it stay bad all of the time?

Check crankcase venting ...

GoFaster - right you are. It was a clogged CCV line. I guess I'm to blame for this one. Back in the fall, I installed a homemade catch can in the CCV line to prevent oil from entering the intake. The design was as described elsewhere in the forums. Turns out, the catch can system was an excellent water condenser. The water was freezing and clogging the CCV. I've gone back to the stock set up for the time being.