Q: 10% of the time, car won't start when rainy weather

I purchased this station wagon from an elderly lady who had kept it in great condition and regularly had it serviced. It had been kept in a garage all those years, so it is in Mint condition! Occasionally it has some trouble starting. When this happens, it is usually in the mornings or when it has been raining. When it doesn’t start, there is a backfire. 90% of the time, it has absolutely no problem starting. I don’t even hit the gas pedal - it just starts like a dream. But on especially humid or rainy days it is finicky. At those times I’ve tried pumping the gas pedal, but that worsens the problem and it runs down the battery, so I have to jump-start my car. I know the wet weather is the issue, but I have not been able to discover the cause and neither have two Volvo dealerships. They say they fixed it, but at the first rainy day, I have the same problem starting it. Have you seen this problem before and if so, what can I do to get my wonderful car in top rate condition?