I have had 2 of these polo's with the same fault.and it turned out to be a problem with valve seating when we stripped the head and inspected the seats there was a definite area that was not seating on the inlets we used engineers blue to confirm this.And this would affect hydraulic lifter problems that made problem appear different at various engine loads,both these cars had quite low mileages and there was no real evidence of the cause. we refaced the seats which were off centre by a few thou fitted new vales and reassembled all has been ok with both cars since.Maybe a bad batch of heads that move slightly