Nova GTE (S16SE) misfiring on part throttle - HELP!

[robg2]

It's a road rally car and I'm doing an event in it next weekend, hence the plea for HELP (sorry!).

I've owned the car 11 years so know it inside out. It's got a standard E16SE which to date has never had any issues.
After a rough & dusty rally in April, it started to misfire - but it's only at light to medium throttle. At full throttle it's fine - it goes like the clappers relatively speaking. It's also ok at idle.

I've checked the whole fuel and ignition system as far as I can see, and swapped as many bits as I have access to. But I still can't find the source of the misfire...

I've been able to change the severity of the misfire by:
1. Replacing the fuel pressure regulator (which was a Power Boost Valve) with a standard one from a C18E 1.8 8v. This made the misfire better.
2. Replacing the fuel injectors with those from the C18E. This made the misfire worse again. C18E and S16SE injectors have different part numbers so I would expect them to behave differently.
For this reason I'm tending to think that it's a fuel problem - backed up by the fact that it runs perfectly well at full throttle. But I'm not 100% certain.

None of the following have changed the misfire at all:
- new plugs
- known good 2nd hand leads / dizzy cap / rotor arm
- blanking the EGR
- blanking the cold start and coasting air valves
- complete new exhaust
- cleaning the air filter

I've checked the continuity of the whole fuel & ignition electrical circuits and all the vacuum hoses and the throttle body and can't find anything wrong.

Can anyone offer any advice? I'm struggling...

[mowgli]

is there a split in the flexi pipe?

i had one that was playing up like that, and i swapped the cts, coil & ign ecu at the same time, which fixed it, so i was none the wiser, but it worked.

[robg2]

is there a split in the flexi pipe?

i had one that was playing up like that, and i swapped the cts, coil & ign ecu at the same time, which fixed it, so i was none the wiser, but it worked.

The flexi between the air flow meter and the throttle body? There are a couple of cracks but I'm pretty ceratin none of them go all the way through. The take off for the coasting & cold start air valves also come off the flexi, but blanked off these make no difference...

[robg2]

It's this BTW...