Here is a heads up for any one battling an intermittent miss on the JZ, and I am certain on others. Here is what I found, and a quick as possible explanation of why these little relays can become intermittent. The relays are an electro mechanical device; on the electronic end, there is a coil, on the mechanical end, there is a hinged steel plate with a set of springs to hold it open and a flat spring with a contact on the end of it that is pulled in to make contact when the coil is activated, there are two contact positions on these relays a NO, (normally open) and a NC (normally closed). Contacts on any relay depend on spring tension to hold pressure on the contacts and to provide a wiping action on the contact face; this action cleans debris from the contact face and ensures a low resistance connection on them. Here is what I found; the travel on the hinged steel plate or the position of the contacts themselves is not sufficient to cause deflection of the flat spring and moving contact, in other words the contacts are just barely touching and there is no wiping action of the contacts. These relays will still make a click, they may test ok and they may work most of the time, however they will always drive you nuts trying to find the problem, as it did me! Hope this helps someone else.

 

Thought I had found it, 230 miles later the same problem is back.

 

Honestly, those little plug-in relays are junk from the start! DC current causes more arcing than AC so the contacts don't last, especially if they're made in China! It's also possible you have an intermittent circuit in the signal wire causing the contacts to bounce, causing premature burnout.

Check the BOTTOM of your underhood fuse/relay box for corrosion.