8th gear has fewest teeth to take the load and is the point where the chain tensioning function aspect of the derailer is maxed out, 7th is right next door. Put in first gear and ascertain if you can tighten the chain by removing a link or two. Might solve the problem. Also look closely at the alignment of the chainring to the cassette. Perhaps the chainring is a bit inset and should be spaced outboard a bit. Dont overlook the obvious, inspect everything very carefully. The problem, and solution, should reveal itself with careful inspection...