This definitely does sound like a build-up of carbon and soot inside the intake system of the engine. Believe it or not, this is not an uncommon problem and occurs partly because of the emissions controls forced upon engine manufacturers.

The diesel engine in your car features an EGR (Exhaust Gas Recirculation) system which means the engine inhales a portion of its exhaust so that it goes through the engine twice and is burned more completely. That's good for emissions, but it means that exhaust soot is being consumed by the engine via the intake system.

Combine that with the oily fumes from the crankcase ventilation system (which also feeds into the intake) and you can soon wind up with a black, sticky, sooty glug that blocks the internal airways much as cholesterol blocks human blood vessels. In some cases, the only way to fix this is to remove the intake system and manually clean it out. This can be time consuming and expensive, but could easily be why your check-engine light is being triggered.

The other bads news is that this could happen again over time. The solution to that is to fit a quality air-oil separator (also known as a catch-can) which keeps the oil fumes out of the equation.

This model Challenger did have a history of engine overheating, largely due to poor surface preparation between the cylinder head and crankcase, allowing coolant to be consumed by the engine. Mitsubishi made production changes that resulted in a smoother head-gasket mating surface, but that's no help to owners of the earlier version.

In any case, that probably shouldn't affect the transmission and if it's dropping into neutral of its own accord then something – possibly electronic – is going wrong., It's unlikely to be a limp-home response as being in neutral doesn't allow you to limp anywhere, let alone home. But it could be a legitimate response to a transmission heat situation. Have you checked the level of transmission fluid?

A scan of the vehicle might throw up a clue, but it could also easily be something as simple as a transmission selector that is poorly adjusted and allowing the gearbox to select neutral on rough on uneven ground. Another possibility is that the transmission is, indeed, getting hot as a result of the engine temperature creeping up. The transmission cooler is part of the engine's cooling system, so the two are definitely linked.

It doesn’t sound correct, but I would talk to the Roads and Marine Services people in NSW. They are the ones who would have made those regulations, if indeed they do exist.