I started by changing the temp sender, it had failed, then the throttle position sensor. Still the same, new throttle cable, removed cleaned and tested manifold heater, had throttle stepper motor tested by injection specialist, found nothing. I cleaned and lubed the rest of that section. Had car Mems tested, failed temp sender.....again, replaced. Had ecu tested and was told it could be wiring degradation. Tried making a new EFI wiring harness but canned that idea when i found NOS harness in France of all places. I put that in and lo and behold ....... no difference.
I've been tempted to fit aftermarket FI, but thats even more money. The amount I've spent, not including my labour, I could have bought brand new twin SU's, fitted and tuned, and had money left over for a beer or twelve. It's got to the point where if it burst into flames on the highway, I'd just sit back and watch. It's drivable ok but only with the idle adjusted manually to a set RPM, but thats a pain when cold and in some situations.
wow, that's a real pain you have had there. I've never had anything like that thankfully, and have found the FI on the SPi to be really good. I have had some faults, but easily remedied. Can't think what on earth is going on with yours that after all that it is still the same 
Yop,
thanks for reading - I have just got my jap mpi / spi year2000 mayfair back to running
after renewing all the vacuum hose connectors
the first couple of outings were good
but today this is what happened
when I declerate the car holds the same rpm for a long time and slows down very slowly if you see what I mean
then when you come to a standstill the revs go very slowly from 1300 to 1100 and then to 900 and occasionaly it just stalls
I don't think the vacuum lines are at fault anymore, the rich running has stopped and the plugs are burning tan
could it be oily residue and carbon deposits left in the piston chambers ??? (I doubt it)
can't think of what to check from here
In relation to the OP's issue above, if it was me I would be looking at stepper motor or throttle linkage issues.
Personally I would do the following. Clean and lubricate the throttle linkages, making sure there is no build up of gunk in the throttle body area, and also whilst there check that there isn't play on the throttle spindle which could cause random mixture issues. Also pop the stepper motor pin out (rubber boot off and it just pops out) and clean and lubricate the hole and the pin. Check the throttle cable is correctly adjusted too. The gap on either side of the lost motion linkage should be equal.
If still no better, then I would wait until the engine was idling at a nice level, say about 900-1000 RPM - should do that as the engine gets about half way to being warm, then disconnect the stepper motor connector. Having done that, the only things that can influence throttle position are the mechanical aspects - throttle pedal, throttle cable, throttle linkage etc. If the problem goes away then that points to the stepper motor, which can have issues with dry solder joints or failing motor coils.
The above suggestions assume there are no vac leaks anywhere, the throttle position sensor has been checked and is working well with no 'spiky' readings over it's travel (usually they wear worst around the idle / part throttle position) and the O2 sensor is known to be working well with good voltage swings at a reasonable rate of change.
I can understand why people dislike the FI minis, it's a different ball game that's for sure, however it is a very good system (partly let down by poor Rover wiring) and in my experience is every bit as, if not more reliable than 'modern' FI systems. Lets see how modern euroboxes are doing when they get to 20+ years old - I expect they will all be suffering engine management issues a plenty!
Really sympathise with peter though, I wish there was some way I could help you sort it out!
