Very possible is that its shifting (slack) thus changine everything. I had a Nexus long ag you could fly it and the tracking was perect. But after you sat it down and hover again my tracking was out of wack.
Check:
Thrust bearings installed backwards
Loose bolt in head
Check Head Block Condition
Lube & chk. rubber dampners
Bearings In Blade Grips
Swashplate wear