@portland_bill has provided some solid advice for a quick check.
In the first instance push the garage about this, I would not do much until you know where you stand with them.
@Beach is the car revving in cycles like this the RPM is cycling up and down in idle? Can you describe this some more or attach a short video? This is important information.
If you have to do this yourself I would do the following two tests from this webpage and report back - they are quick and easy:
Brake_Booster_Tests
- Basic Operational Test
- Hard Brake Pedal Test
Some additional background information:
The brake booster line is indicated here in orange and is the solid black tube that runs from the top of the engine (near the throttle body) to the brake booster. If air is leaking through this line into the engine it may overfeed air to the engine. As it is a vacuum tube it is quite hard (as I remember it).
@portland_bill suggests pinching it off to check by stopping a leak if it is there. However, you need to be careful so as not to damage it, like with a thick cloth on it and some vice grips. Don't commit if you think you might damage it.
If pinching is not possible you could disconnect it, start the engine, let it idle, block the port to the engine (that you removed the pipe from) with your finger and see if idle improves (as this removes any potential leak). If it does there is something wrong on the booster line.
For reference the brake booster is this part and the pipe you wish to block goes into the large circular black part that houses the diaphragm: