I have a problem with 5th also, all my gears were nice and smooth before it went in to have a new down pipe fitted, now its awkward and sometimes takes a few goes to get in... I should have taken it back at the time, but i've kinda left it too long now.
I havent even looked at it yet, but just read this thread and thought i'd mention it, I would assume they moved something or even broke this part while doing the downpipe. does that sound likely ?
Thanks