There's some things to check:
- Have you checked the engine for any oil leaks? A hot engine & dripping oil onto hot parts can create quite a strong fumes smell. Check the fuel filter to see if there's any leaks around that too.
- Is your exhaust & manifold in good condition? (No cracks, letting fumes escape where they shouldn't!)
- It could be DPF related, I presume your car has one? Maybe your car is doing a regeneration of the system, or the DPF filter could be blocked? You should have an indication of the latter by a warning light on the dash, or a potential loss of power or fuel economy. A regeneration can cause a burning smell though.
If you do a lot of around town driving or driving at low speeds, you need to give the car a good run on the motorway or at higher speeds to clean the system out for at least 20 minutes from when the car is fully warmed up. Given that the smell your experiencing only seems to arise every so often, I'd be inclined to think its related to the DPF. If its a regeneration of the DPF system (Cleaning soot out of the filter), then the smell is actually normal providing it goes after the process is finished. Give the car a good run on the motorway or a dual carriageway to see if it helps & don't be afraid to put your foot down every so often (
) If you get no smell of fumes for a while after doing this, do the same again when you do get the same smell again to see if it helps. If it does, then its your DPF regenerating (Which is normal, but it needs time to fully complete - 20 minutes at higher speed from when the car reaches full operating temperature).
Failing that, I'd be bringing it straight back to the dealership of course.