Fiesta mk4 1.25 automatic power loss?

Hi,
Was wondering if anybody could help? I own a 'p-reg fiesta ghia x 1.25 automatic'. (The CTV gearbox) About 6 months ago it was serviced and passed an MOT thesame weekend. A few days later i seemed to lose 90% power when accelerating. i seeked garage advice and was told the gearbox had gone and a replacement would be expensive for a fairly decent one. A few garages said the same yet this week a guy picked it up to change the gearbox over to another automatic. He then phoned and said the gearbox is ok and it as just the wrong gearbox fluid. apparently it was too thick. He changed it all over and for a few hours the car has been fine.
Now though, im having the same symptoms as before, a few weeks before it went last time. Basically the car would drive ok and every now and again whilst accelerating (witout moving my foot position on the throttle) the car would just lose power then quickly pick up again. this starts to get more frequent and would even cause big problems when pulling away from a junction or something it'd start to pull awayand then nothing. Its quite dangerous if im half across a junction or round-about. Was wondering if it could be a fuel problem, or maybe a computer fault. its really bugging me as its still got a bit of value in its otherwise good condition and i need to sell the car but in good working order. haha. Thanks for your time. Please get back to me a.s.a.p.
Regards