My 2001 S80 has started to show some problems. The automatic transmission finds it difficult to shift into 2nd and 3rd gears and often the rpm goes higher than usual for it to shift successfully. But the problem is erratic and it often becomes easier to shift if I press harder on the gas pedal. No problems downshifting or in any other gears. The transmission oil was recently changed as it seemed burnt. The problem then seemed to go away but came back again after a couple of days. What steps can I take and what would be the cheapest way to diagnose and solve the problem?