The hardest thing to explain on a telephone is that a fault which is not currently happening is very nearly invisible.
An electrical fault is a measurement: this wire should have twelve volts and has nine, this earth should have no resistance and has two ohms, this connector is fine until it is warm. All of those need the fault to be present while somebody holds a meter on it. A car that misbehaves every third afternoon and is brought in cold on a Sunday morning has told us nothing.
So three things help more than anything we own. Bring the car with the fault happening if you can, even if that means driving around for twenty minutes first and arriving hot. Bring notes: when, what weather, hot or cold, moving or standing, and what else was switched on. And if the fault only shows up after the car has sat overnight, say so, because that is a different booking and it involves leaving the car with us.
The clinic on Saturday mornings exists for exactly these cars, and half an hour there usually tells you which of those three situations you are in.