Simon,
I am 6'2", bought my America after riding tourers for years.
I had the same problem, did the National Rally after having the bike for 2 months (20hr ride), hurt in places I didn't know could hurt!!, but it cured the problem!! So I came to the conclusion it was all caused through being to tense!! So don't buy any mods, until you have learnt to relax a little first, then see how you go.
P.S. Has Ray mentioned 'Dent' yet?!!!!
