Temperamental 1980 Volvo

I have a 1980 Volvo with approx 90K miles. It used to belong to my grandma and I inherited it about 8 years ago when she lost her license. It came from California where it lived in a garage except for weekly grocery trips and the occasional joy ride.
For the past several years, every time the weather is cold and damp it has trouble starting/warming up. It will start, then die several times. After I get moving it will stay running if I put it in park or neutral when I am stopped, although it still sputters a bit unless I keep the gas on. Once it gets going it is pretty much OK for the rest of the day.
During dry or warm weather it starts up fine. Every time.

I have an excellent mechanic (he meticulously cleaned out the gas tank/fuel system by hand when it had disintegrated in the gas tank - or something- just to see if he could do it, and he did). He has addressed this several times by adjusting the fuel mixture, but it just keeps happening. When I take it back after it starts happening he tells me the fuel mixture is all screwed up and I wonder if it is readjusting itself or what?

Does anyone have any idea of what could be happening? Is it because the car grew up in California?

Has your mechanic diagnosed the start injector and thermal timer? Has he/she checked the start injector for leaking.

This engine comes with either the Continuous Injection system or the LH Jetrronics system depending on the engine configuration. The CI system does not have a computer and the mixtusre is mechanically adjustable. The LH Jetronics system is computer driven using a oxygen lambda sensor to adjust the mixture. Could you verify which system you have?

Hope to help.