[Steve Tinker]

Piers...
You don't mention your cars mileage, but if its high and maintenance has been neglected, piston rings and or bore wear usually cause the symptoms you describe.

1) For a quick check of the AOS, can you remove the oil fill cap easily when the engine is idling - if there is suction there and cap is hard to remove, its a good indication of a failing AOS.

2) Did you thoroughly clean out the plennum chamber (where the throttle body is situated) when you replaced the AOS?? There has been quite a few instances where there is a lot of old oil resedue coating the inlet system after a bad AOS.
There has been instances on this board where new AOS's have failed almost immediately.

3) Has your oil level dropped since you replaced it?

4) I'm assuming you have no intermix in the oil or coolant resevoir.

5) Do a search on AOS and get ready for an hours worth of reading.....

If all else fails, you should do a compression or leakdown test to check ring / bore wear.