It's hard to say...
What macro are you using to assist on mobs etc does the macro have it's own movement routines?
What are you stick settings? (found in your [MQ2Melee] section in servername_charname.ini in root MQ dir
Sounds like something is trying to move your char and it's conditions are too definitive and background windows with lag it has a hard time meeting the conditions. Try looking at
MQ2FPS - MMOBugs Wiki
But you are asking for an answer to an equation with too many unknowns.