My 91 Fiesta is a te,permental old beast!

Randomly out of the blue boosting @ 8 psi about 3500rpm it slams out starts jumping around all over the place. It keeps doing this but only allows you to boost to 5psi and does it all over again. Turn the engine off and try again and its fine!!! Sometimes wont happen for a couple of months.
I've got escort cossie plugs which have been gapped to 1mm. I wouldnt say its backfiring or blowing the spark it feels like one of these 3 things!

1. Fuels cutting temporarily under load.
2. Ignition pack is playing up.
3. MAP sensor.

I havent gotta clue??? It runs smooth on idle, a bit lumpy @ 2000-2500rpm.

Hope ive made sense and please try to help me!!!

Cheers

 

Amal valve? Heard they can cause alot of inconsitent boosting when they pack up, i dont have one on my fiesta engine tho so dont know much detail.

 

throttle position sensor cured 1 ive just had in for similar problems