Brand new leads can still be faulty. Last time I bought champion leads, 4 out of the 6 were bad. The easy check is to take them off one at a time (a pain on the EF's I know) and check them with an ohm-meter. Generally it is accepted that 1k ohm per inch is servicable but for my liking regardless of length 6-8k ohm is my limit.

Even if they check out ok this does not mean they are perfect as they could still be breaking down under load.

cheers
Scott