2009 Subaru Forester Shuddering

Hi,
I put my car in for service and the mechanic replaced the brake pads and machined the discs.
We picked the car up and it was fine driving the 20 minutes home. It sat in the garage for 20 minutes and then about a mile up the road going approx. 40mph it started shuddering so I let it slow down and it only got worse. Pulled off the road and got out to look at the tyres and could smell a burning rubber smell. Drove it to my work another few miles up the road going really slow to try to not have it shudder so much, called the mechanic who picked it up on a flat bed tow truck.
He test drove it a few times and it didn't shudder again all day. He also checked the whole car over as well as another mechanic and couldn't find anything. So I picked it up in the afternoon, drove it a mile down the road and had dinner. About 40 minutes later, start driving it home. Get a few miles up the road and it starts shuddering again. Turn around and drive back to the mechanic and the rest of the trip it didn't shudder again.
Mechanic took it home and then it only did it the next morning again. He checked everything again and has no idea what's wrong with it.
Please help