Random cloud of smoke

[AzzaClazza]

2008 Astra with 93k.
At random times while driving the car will suddenly lose power and belch a massive, highway blocking, cloud of blue/white smoke. After a few seconds it rights itself and I can continue to drive normally.

It can happen at any speed, accelerating or cruising.

I suspected the PCV valve so replaced the entire rocker cover. The old cover had quite a lot of tar build-up.
The problem has not gone away.

I noticed that there is a LOT of vacuum at the oil filler cap. When I open the cap the car runs roughly. The PCV valve is brand new so I've tentatively ruled that out. I suspect the vacuum and the smoke are related. What would cause such a strong vacuum? I thought there was only meant to be a slight vacuum.

Faulty intake gasket? I've read the suggestion of a blocked breather but I don't know there is one. There are no other lines to the rocker cover. Could a vacuum that was too strong cause this?