Have had exactly the same fault which will eventually lead to a non start condition. Follow the harness out from the ECU at the offside inner wing. It goes towards the bulkhead then over onto the engine, just above the alternator. If you pull the harness out a bit, you will see there are two black/orangey brown earth wires which go onto a large ring terminal & 13mm bolt above the alternator. These are the main management earths for the ECU etc. These corrode and the wires fall to bits, causing speedo and lambda problems as you mentioned. If wires are heavily corroded, I strip them back and repair them and re-site these earths onto the bulkhead bolt for the cylinder head earth strap that you see close by.