Like the fella above says, get it looked at.

Did you build the engine yourself??
Check you aint got the timing belt out by one tooth on the cam pulley. If you are out the car will be difficult to start but will run but really lumpy and low on power.

Your starting problem when warm could be a number of things ~ Alternator on its way out, battery not holding charge and quit common on the old RS Turbos is the wiring breaking down as it gets old and not being able to draw anough current to turn the starter motor.

I have a Ally actuator same as you from Burton, rated 19 - 22 psi (I think) and its so difficult to get set up correctly, in the end i just settled for the 22 psi, which is more fun anyways.

Your probably cutting out at 90mph coz your hitting over boost, your new chip may be set to cut the engine if you go over something like 22 psi (Although i'm not all that clued up on chips).

Your not far from where i live, Try giving either Engine advantages (essex) or Race Power Motorsport (RPM [Gravesend]) a call.
Both should sort you out.

Matt