UK Car Repair: problems with 06 plate corsa, air flow meter, crank shaft

Question
Hi I seem to be having major problems with my 06 Corsa, it is has been in the garage for 5 weeks now adn the garage dont  seem to be any further along with, it began losing power 5 weeks ago, it starts up fine and goes perfectly for a couple of hundred yards but the the power goes and I am unable to pick up any speed, so far the mechanic has replaced the crank shaft sensor, air flow meter, coils, spark plugs and now he sent off my ESU which has come back (alledgely) with no problems. can you help please or do you have any sugestions.

thanks

Answer
right, I can diagnose this but you will have to be really specific.

answer these questions as fully as possible:

1. when it first lost power, desribe what led up to it struggling, last 3 uses of car etc.

2. how did it lose power: gradually, suddenly, engine light on? any other warnings/symptoms?

3. when the car is struggling is there any misfiring? how is the temperature gauge at this point?.

4. is the car always used for short journeys?


- my guess- fuel cap is blocked, not getting air into fuel tank-vacuum builds up and fuel can't be delivered as fast as needed.

first attempt at fix:
**drive car with fuel cap removed**
(in a straightish line, not round fast bends as fuel may spill out) - does it keep its power like this?

if so - fit new fuel cap/

or... clear the blockage in fuel cap by washing it (remove from car first, soapy water+scrubbing brush, dry it thoroughly)

otherwise if fuel cap makes no difference answer the 4 questions as thoroughly as possible.