This sounds like what happened to my brother with his Scimitar.
He has recently converted it to electronic injection from the old (70's) carburettor.
This was not an easy project by any means, but being a tormented genius he has persevered with it and now it's good. (he's actually just a genius, but I torment him..
)
When the ECU was "learning" he said that each run did get slightly better for a while and then began to go down hill! This proved very frustrating and after exhaustive investigations, it proved to be a faulty sensor, sorry can't remember which one (possibly TPS?). Once the sensor was replaced by the supplier of the bespoke conversion kit, the "learning" re-commenced and I believe things are now very good.
So maybe a faulty sensor somewhere?