[davepl]

I think it's a longshot, but it popped into my head, so: when you load the engine it torques up against its mount. If that's where its primary ground path is, it'll disturb that.

That should only be the case if you don't have proper ground straps. Do you have at least two copper/aluminum braided ground straps from the block to the frame?

Since it's not the original engine, it can be missed.