Driving in limp mode

just had full service and cam belt done.
Had a problem with inconstant loosing power as going up the gears. Stopping the car, turning off re-set computer allowing me to drive normal mode again, until i needed to do it all over again due to loosing power again.
Asked garage to check this out while servicing it.
They said it was turbo, generic veins and used a new product to flush and clean veins/turbo and injectors fully.
When i received the car back again, it worked fine (although not perfect) for one day only.
The car is now on permanent limp mode and does not allow me to drive harder than 50 miles per hour after a long stretch of road with no hills.
Also if i hold clutch down and put on the gas while driving, i get a nice puff of black smoke out of exhaust.
If its still the veins, im not covered by cover if its simply a broken turbo, not down to wear and tear im covered.
The car has done 80,000 miles.