a freind has w reg classic wrx and when driving the fuel pump turns off and car stops so thinking its immobiliser cutting in can yo remove mmobilser in any way buy changing ecu, or not as simple as this, its the factory immobiliser with blue fob

 

does it have an alarm system? Maybe a loose connection? I had what sounds like a similar problem a few weeks after having a new Clifford alarm fitted. Car cut out in outside lane of M6 and apparently it was due to a loose wiring connection into the alarm control unit!

 

Oh yes
got an 02 'blob eye' and the from new sale agent had fitted a Subaru recommended alarm & it would cut the ignition at all sorts of times, in a garage - on the road - outside the pub (nasty comments from Escort owners) changed ECU 3 (yes) 3 times and still didn't cure it.
Took it to SVA Imports near Folkestone (nearest good Scooby agent to me, great guys and do excellent fair priced work) found out it was a wiring problem upstream of the ECU control, and it was cooking the wiring, got close to a fire apparently, had to rip the whole damned thing out and buy a very expensive 'Caterham 10'
It's worth getting a good lecky to have a poke about, this prob will not show up on the analyser. so they change the ECU (again & again)
Good luck mate, still got the best car in the system though haven't we ???
best regs. AL.W

 

Check the fuel pump wiring, some of the classics had issues with the wiring breaking under the back seat area if I recall