Cruise Control On C3 1.4hdi

[Cupcake]

Having had my C3 for a few months now, I recently had it off road as the engine mount broke. This has now been replaced by my mechanic. He looked, but could not find any damage which may have been caused by the engine collapsing. However, getting back in the driving seat and taking it daily up the motorway, I have noticed that once I set the speed for the cruise control, it does tend to drop a few mph. Not sure if this should happen as my previous car with cruise stayed at the same mph it was set at. Also, if I put my foot down whilst in 5th, it seems very sluggish to pick up, seems there's a bit of power loss. I took it to have the exhaust checked today as I thought it sounded noisy on the motorway coming from under front of car, but that all seems fine... any of you technical bods have any answers???