Distributor chuffing?

[RS13]

Hey,

My car has been missing as of late, basically missing momentarily every few seconds, I thought it may have been leads, but changing/fixing them/cleaning out the distributor didn't help. I popped the bonnet before, and noticed that when idling, the distributor has 2 little holes on it, and when the car misses, the lower hole puffs out what I can only assume is crankcase pressure?

The car is perfectly fine until it warms up, or if I let it heatsoak a little.

Thoughts? Why are there holes in my distributor? When the car is warm, it does it regularly, like, every 2 or 3 seconds.

Cheers.