T5 hesitancy on full boost
 

I have a 1996 850 T5 manual and it spoils up nicely to around 12psi with no leaks (just inside the white of the gauge) but if I hold it there for more than a few seconds it seems to cough/hesitate, almost like a fuel cut-off - which shouldn’t happen around those pressures.

Has anyone experienced something similar and could advise?

7psi max is standard so firstly your car has the boost raised with most likely a remap. Any ideas what remap it has?

Fuel pump - may be fine at stock psi and slightly more but when demanding additional loads to deal with the demands of a remap it may not be able to provide consistency. May be even a dicky fuel pump relay.

What are your ignition components like? (HT leads, plugs, distributor and rotar)

Have you got any way to measure the manifold pressure with a tool that is fast enough to log it at least at 100ms resolution? This would be something like a PicoScope oscilloscope and a pressure-transducer like the WPS500 into the compressor outlet pipework.

What you are describing sounds a little like a compressor surge, which is where the pressure-ratio across the compressor has been mapped too high for the mass airflow that is passing through the compressor (i.e. insufficient wastegating in time to prevent the pressure building too high).

The effect of surge is to momentarily reverse the airflow across the compressor, and knock-on effect of mapped fuel delivery to go with it.

It sounds like fuel cut is kicking in. I had this when I bought my 850R, which would go like a scalded cat when booted, and the boost gauge went to the end of the scale, which was fun for a few seconds until the fuel cut kicked in.
I wound half a turn onto the wastegate actuator to lengthen the arm to make it open a bit more (as someone had been there before to increase the boost). I also cleaned the MAF with a tin of CRC Air Sensor Clean.
Now the boost gauge stops just short from the end of the scale, which it's supposed to do on the 850R, and I don't get fuel cut any more.