I still miss my V6
Join Date: Aug 2008
Location: United Kingdom
If the engine runs at all, the ECU is not faulty. Yes, an input pin could possibly fail, but then a fault would be logged. Anything else and the thing just wouldn't do squat. Long and short of it; don't waste money on an ECU.
Exhaust won't be having an effect on fuel economy, and you've replaced the lambda probe.
Have you looked at that old gem known as BBoB? Resistance in this part of the engine harness can cause your symptoms, though I'd have expected it to worsen severely over the length of time you've had this issue
GialloEvo94 will be able to provide a link to a comprehensive guide on the BBoB subject, I can't seem to find a thread he'd link to.