My engine misfires sometimes, and I can't figure it out and neither can
two workshops I've been to -- but the details are pretty specific so
I wonder if anyone here can tell me where to look next !

My car runs perfectly for about the first 15 minutes of driving. But
after that, it runs alright at low throttle, but misfires as soon as
I put any appreciable load on the engine. It /feels/ like it is
under-fuelling but that is only my feeling; the other possibility
is of course that it's dropping one of the cylinders.

In lower gears I can get about 3/4 throttle and some acceleration
before the misfiring starts, but in 5th it happens as soon as I go
past about half throttle.

Yesterday I managed to get it exactly as it switched over from
working great, to being sick. I had my foot flat and was up to
7k RPM in third gear, and then blam, no engine for nearly a
whole second, and then it picked up again at about 6k RPM
(but I instinctively pressed the clutch and stopped giving it gas).

It /must/ be something overheating and then stopping working
properly. It certainly /feels/ like it is under-fuelling, when it is
running badly. But I cannot think of any component that would
overheat and cause under-fuelling (surely the fuel pump can't
get that hot?)

I do not think it is a wiring problem because when I stop the
car, it always idles on all 6 cylinders; if a wire were expanding
or a spark plug failing or something, then I would expect it
to continue to fail until it cools down.

If I had a dodgy air-flow meter or a badly gapped spark plug
or something like that, I would expect it to always run badly.

At my most recent workshop visit, they changed the cambelt,
did a compression test (successful), and informally cleaned
the injectors. This made my problem not so bad (ie. it would
run for a few more minutes before going sick, and then it
would tolerate a bit more power before misfiring once it was
sick).

My engine is a Nissan straight-6 , turbocharged, intercooled,
with multi-point fuel injection and direct ignition (ie. no
distrubutor, and one coil pack per cylinder that sits right on
top of the spark plug). Aftermarket ECU, BOV and boost
gauge. Redline is 7500rpm but I've had it up to 8500 (in the
distant past) with no trouble. The driveline is rear-wheel with
LSD and 5-speed box.

It used to run perfectly on all the same hardware as I have
in it now.

Any ideas?

Is it possible for a mechanical problem in the driveline to
cause those symptoms? (just a random though I had
yesterday, since my gearbox is pretty tired and rattly).

My current plans of attack from here are:
 1) Attach a fuel pressure and air/fuel ratio gauge
 2) Wait till the engine is really hot, and then swap in
some cold coil packs (to eliminate them as a cause)
 3) Swap in a set of known-good injectors
 4) Re-fit the default ECU and check for diagnostics