I’m amazed that you managed to find such a convoluted reset procedure, even if it only provides a temporary fix. You may well be right that this behaviour is linked to heat build-up, especially if it happens after several hours of driving.

It’s interesting that you mentioned the engine sometimes goes into limp home mode, because the transmission’s behaviour sounds a lot like it’s going into limp home mode, too. Limiting the gears the vehicle will select is a classic symptom of limp-home mode, in fact.

The real question then becomes, what’s causing it to do so. Limp home mode can be initiated any time the vehicle thinks that continuing to drive at full power or speed will do further mechanical harm. As such, there’s a huge range of things that will cause limp home mode to engage, including a leak somewhere in the turbocharger’s plumbing, low coolant level, low oil pressure, low fuel pressure, a faulty sensor and many more.

So the first job is to scan the vehicle and see what fault codes are thrown up by the computer. From there, you can home in on the actual problem and fix it. But without those codes, you’re kind of flying blind.

The usual procedure for flushing a cooling system is to drop the existing coolant (usually by disconnecting the bottom radiator hose) fill the system with distilled water and whatever flushing chemical the workshop uses, restart the engine, drive it around for a short time until everything is at operating temperature, dump the water and flush mixture and refill the cooling system with the proper coolant.

You can also simply flush the radiator (and not the rest of the cooling system) by disconnecting the radiator hoses and running a hose through the radiator, usually in the opposite direction the coolant flows. But this is a much more simplistic flush and doesn’t change the condition of the engine crankcase’s cooling passages.

It’s possible that the flushing process has loosened some rust or scale that then blocked the cooling system somewhere else (the thermostat is a prime suspect) and if that’s the case, the process needs to be repeated until the water coming out of the cooling system is nice and clean and not tainted with rust or scale or anything else.

Revving the engine to get the flushing mixture to do its job is not really a problem, but could also be achieved by actually driving the vehicle (which would also get it up to temperature faster). But, really, provided the temperature gauge hasn’t been allowed to move into the red zone, you shouldn’t have damaged anything. But you do need to know why the operating temperature of the engine is now higher after a cooling system flush.

This type of thing is certainly not an uncommon fault, but whether it’s more common in Toyotas is another question altogether. Certainly, Toyota has not recalled the vehicle to fix this problem, nor am I aware of a service bulletin (much more likely than a safety recall for a dud head unit).

But the good news is that there are specialist companies out there that fix problems like this one. You remove your head unit, send it away and it returns fully functioning and tested, ready to reinstall. Flickering and dropping out are both common faults in modern head units, but both can be fixed by the right specialist.