Have you tried changing the order of the clustered index. You say you did try this index:

(IntDate ASC, CustomerID ASC, PartitionID ASC) ON[PartitionedTables](PartitionID)

But have you tried putting the PartitionID fist? :

(PartitionID ASC, IntDate ASC, CustomerID ASC) ON[PartitionedTables](PartitionID)

I'm very curious what happens when you try this, so please post the result....