Hello world, I've recently been having the same problem as the OP (skipping in the 9 tooth) and after some months of tweaking the company that put together my bike swapped the Tiagra rear shifter for a Capreo one (apparently they've just released a larger-capacity version). Despite being technically over capacity, the derailleur works nicely with my 53-38 up front (9-26 cassette) and has more or less eliminated the skipping. Sure, if I'm massively overheard and really push it into a headwind I can still make it skip, but if I cycle sensibly it's no longer a problem.

See also this thread.