It could be: Fuel pump/fuel pump relay/fuel filter.
It could also be faulty spark plugs/HT leads/coil
Did they check all of these ?
I'd bring it to a place with a diagnostics machine immediately when it feels like it's losing power. Get them to plug in the car to it and check the sensors. As a lot of intermittent problems are with sensors failing or about to fail, this could be a good first step and cheaper than replacing expensive parts that are potentially fine on a trial and error basis.