MAF sensor was the first thing to come to mind....
15°-18° BTDC seems like a bit much, honestly. 14° BTDC is more common, but it's still on the ragged edge for most motors. I usually ran 12° BTDC myself, to get out of the hole faster.
Spark plugs been changed lately?
Sure you're not losing fuel pressure?
Is the damper actually showing the correct timing?
How about the strength/color of the spark?
Has the TFI module been replaced?