Nimh Cell Problem?
 

Was running a Slash yesterday and it seemed like it was much slower than normal. Checked the voltage of the 6 cell sport pack and it was only 7v, charged it again and it went to 8.5 but by this morning was back at 7.3v.

Should it lose voltage that quickly after charge, am worried there's a dodgy cell but don't want to rip apart the pack to check individual cells if possible?

check the voltage across each cell. that'll tell you which of the 6 is faulty.