2005 Honda City Trnamission problems

Hi,
I have Honda city vario 2005 that I bought without driving it. I am facing problem that sometime car does not drive immediately in D when I Start and this problem does not occurs mostly its gives knocking sound. It also happens on steep climbs on slow speed. Rest of time car runs fine and respond to acceleration pretty good. I have changed its Transmission oil from Honda company workshop, 8 plugs, air cleaner, also changes its engine oil. The problem still exist. Please guide.