[Ronbo]

Depends on where your connecting to...

Are you using the car side of the cut off switch as one of the connections?

Cliping directly to the bolts on the battery or the lugs?

Just to eliminate it try disconnecting the car ground lead from the switch and hooking directly to the battery post.

Don't assume the cables are good as the crimped cup in the terminal can still get corroded or loose since their not soldered on. (although Kirkham does a good job of sealing the terminals, you may have a "soft" crimp)

Keep in mind you also have a very small surface contact area using lugs to the battery so it needs to be spotless.

Another thing to check is clearance of the battery bolts; take the lugs off and just run the bolts into the battery to see where they bottom out. (how much gap is left)

This sounds like a connection problem or a bad cut off switch. (depending on what your "jumping" to)