This could simply be a case of the engine’s idle speed being set too low. If that’s the case, even though it’s firing on all four cylinders, it can feel rough and lumpy as though there’s something major wrong. Then, as you increase the revs, the engine smooths out and feels normal again. Even a worn or broken engine mount can amplify vibrations at idle, as can an exhaust leak.

A good mechanic will be able to tell you quickly whether this is the case or if, in fact, the engine really is dropping a cylinder at idle. If the latter is the case, you may have a dud spark plug, ignition wire, sensor or even a blocked or dirty fuel filter. Again, most mechanics won’t have a problem tracking this one down.

The fact that the scan doesn’t throw up a fault code could be an indication that it’s something pretty fundamental at fault here. Have you checked the car’s earth points and the tightness and cleanliness of the battery terminals? Apparently simple things like this can easily cause a random misfire. How is the condition of the battery and the charging rate? Modern engines hate low operating voltages.

But don’t rule out a problem with the fuel system, either. You might have a faulty fuel pump or a dirty fuel filter or even a dud injector that’s causing the glitch. Don’t forget to check the wiring to these components, either as a simple loose wire could be the problem.

If you do need to dig deeper, you could try checking the engine’s compression and the clearances of the engine valves. Even something like a cracked cylinder head or faulty head gasket can cause a misfire that shows up at, say, a particular engine temperature, but not at other times.

It sounds like the new head gasket isn’t working properly and the car is still overheating. Or, the root problem wasn’t the head gasket in the first place, or that the mechanic missed something crucial that allowed the new head gasket to fail almost straight away.

Was the cylinder head checked for straightness? Was the cylinder block checked for cracks? A warped cylinder head or cracked block or bore can give symptoms just like a failed head gasket, but replacing the gasket won’t fix them. I think you need to have a deep and meaningful conversation with your mechanic and work out a plan to either dig further into the engine or replace it with a second-hand unit that’s been tested. The latter is probably the smarter, cheaper way to go. Either way, though, you risk spending more on the car than it’s actually worth. Maybe it’s time for something newer, more efficient and safer, not to mention more reliable.