Hi hoping someones able to help me. I've got a 1991 xr2i 2.0 zvh with a misfire.

When you first fire it up its fine then cuts out. Or if you rev it it's fine but when you try to hold the revs it misses in a big way. I've tried bypassing the fuel filter and put on spare inlet manifold with different injectors and idle valve. I've put on spare coil pack new plugs and leads and still no change.
The engine is newly built so obvious all fresh and no smoke or oil going where it shouldn't, im starting to run out of ideas and think it must be down to a sensor or something along those lines so if anyone has any ideas it would be greatly appreciated.
Many thanks Neil.