Intermittent coilpack 1-4 no spark removing lead 2or3 fixes.

[Nevyn72]

The next step is to very carefully work your way along all the wiring in the engine bay looking for wear. This could result in some exposed wire leading to an intermittent short....

[93_Clubman]

You could also check the ECU for water damage, & check that all earthing points are secure.

But first, if you haven't already, specifically check the wiring immediately before the coilpack as there's a known weakness there that's not necessarilly visible to the eye. The wires inside the plastic insulation deteriorate where they deflect just prior to plugging into the coilpack. Apparently the combination of deflecting & not being well supported in this particular spot, probably compounded by alternating high & low temperatures, weakens the wires. It's not immediately apparent to the eye as the plastic insulation may appear intact, but if you move this part of the wiring loom around while the motor is running you'll get a response in terms of idle if the wires have deteriorated.

[manga_blue]

It's all pointing to weak 12V main supply voltage at both coils. 93_Clubman is pretty much on the money. Do everything he says, including all the earths.

I'd also be tempted to replace the engine loom. Otherwise you can mod it by adding a 30A relay drawing from a solid 12V power source just before each 12V side of the coilpack - a bit bandaidish though.

[manga_blue]

Not the wiring harness, just the engine loom. Or at least I thought it would be a good idea until I had a closer look at it. It's more complex and widespread than my NA8 loom. You'd need a patient forum member to get one out whole. A wrecker wouldn't have time to do it properly.

[93_Clubman]

Do all the electrical circuits ground back to the battery area in the boot or are there multiple grounding points in the engine bay?

There are multiple grounding points throughout the vehicle, with a major one in the engine bay as well as some minor ones. Manga_Blue has posted the most info ref earthing points, but don't think they're all listed in one post on here.

[manga_blue]

Just going from memory here from my NA8. Ithink NB8A is much the same.

In order of importance for engine grounding they are:
- coil/injector loom to rear of inlet manifold
- misc bits to bracket below front of inlet manifold
- engine block to chassis from near dipstick to firewall ledge
- PPF to chassis near rear axle

Also check all connections to alternator are clean and sound, especially the white.

[93_Clubman]

My soft top leaks near the windscreen/window junction. Is there a way to adjust the latches to increase tension in soft top which could give me a better seal?

Re soft top latch adjustment:
http://www.miata.net/garage/garagetops.html

If you haven't already, then also clean all the water drain points to ensure that water isn't getting into the car by overflowing from blocked drains:
viewtopic.php?f=28&t=35601
http://www.mx5oc.co.uk/forum/forums/p/270/439.aspx#439

Regarding the ECU, will a relocation help with this problem if it takes me a while to fix the leaks? Does anybody know of this being done successfully?

Don't know for sure if you could relocate the ECU to the glove box on an ADM NB, but there might be enough flexibility in the loom to do it.
For info, the ECU in the USDM NA was behind the driver's seat (LHD). This location may have carried over to the NB. The immediate consideration that comes to mind from the NA would be that you could need an additional earth strap for the ECU given it won't be mounted to metal via metal brackets as per factory.

[manga_blue]

Once again speaking only for NA8 ECU location, I just velcro'd mine into the glovebox. No earth issues, cables long enough.

[93_Clubman]

I pulled back the carpet in passenger side footwell to get at the ECU and it was quite damp around the middle section.

I managed to relocate it to beneath the glovebox
Unfortunately this didn't fix the intermittent spark problem

If the ECU got wet then unfortunately the damage has already been done - try swapping in a known good NB8A ECU.