Ongoing issue, possible vacuum leak or CCV?

[X5Kiwi]

For the last 2 months, my X5 has had the engine check light coming on for a few days, then clearing itself.

Twice it has been in to the dealer to get checked over, both times the check engine light has been caused by a fault on both oxygen sensors (before the cat) and both times they have been looking for vacuum leaks but have not found anything.

They don't believe that both O2 sensors have actually failed simultaneously.

After the second time in the shop, with the mechanic not able to find any faults, I drove approx 500 meters and suddenly got a constant miss fire. I returned to the dealer, and they replaced one of the coils, which fixed the miss fire. They suggested this might have been the cause of my previous problems.

However, only a few days later the check engine light came on again. The engine 'felt' fine most of the time, except under light load and below 2000rpm (i.e. driving up a slight slope in town) the engine would seem to cough a couple of times over about 3 seconds. Every day, on the same hill on my way to work. Also, my fuel consumption has gone up about 15%, without any driving style change. (The dealer did explain that with the computer registering faulty oxygen sensors, it would push the fuel map to full rich on idle, so this might explain this).

Today, however, the miss fire is back, continuously, even at idle. The check engine light is flashing. I called BMW, they suspect "a pipe that runs under the manifold" and have told me to bring it in on Monday so they can remove the manifold and check the "pipe" out.

I'm assuming they mean the crank case vent? From realoem it looks like there is a hose running under the manifold to the oil separator?

So my questions:
1. Does this list of faults sound like some sort of fault with the CCV or oil separator? I do NOT have any smoke coming out of the exhaust.
2. Any other ideas what could be causing these problems? I've already spent over $1000 on diagnosis, with no results.