2004 Kia Rio sudden engine rev up

The car has 1.6L engine. When test with engine diagnose device this is what came up in its error code:-

P1507 = MPS (motor position sensor) - abnormal
P1505 = kick down servo switch

the check engine always light up even after the error code has been erase.
during in idle, the engine timing suddenly increase from normal but after few second it drop down back to normal. This condition will occur a few times but not always. What should I do to resolve this problem? What should I check/investigate/test?