I had similar misfire / poor running issues on my 2007 185 D5 xc90 recently, also related to cylinder 5. See this thread:
http://www.volvoforums.org.uk/showthread.php?t=266079
I had already tried to check wire & terminal resistances and all appeared fine, it only showed up as a fault on mine after everything was thoroughly warmed up.
Totally resolved now after replacing the terminals with part number 30765321, listed as a 'repair terminal' and which contains 2 terminals pre-crimped to lengths of wire. About £8.40 from the dealer.
I used a safety pin to release the old terminals from the plug.
Could be that cylinder 5 is a bit susceptible as the loom is tight in this area - I left some extra length on it when I soldered in the repair terminals.