This is a 1991 Volvo 245 California car, automatic transmission with 145,000 miles. In the last couple of years, the car has acted up in a characteristic way (hard to start, stalling) under three circumstances. I'm inclined to believe the three circumstances are related. Let me describe each circumstance.

Circumstance One. Several years ago on a rainy day I drove about one quarter mile and stopped at a stop light. When the light turned green, as soon as I stepped on the gas the engine balked and barely ran, leaving me slowly rolling into the intersection. By the time I completed the turn, the engine caught and returned to normal. I then travelled about a mile where I parked and ran an errand . About a half hour later I returned to the car and attempted to start it. It would get on the verge of starting, but did not catch on. Sometimes it would catch, but as soon as I gave it a little gas, it stalled. This went on for about 15 minutes. Finally it ran well enough to return home. On the way back, I had to stop at a light about a mile away and the same behavior resumed. It would barely run, which resulted in creeping through the intersection, much to the irritation of the motorists behind me. Again, it then ran normally.

Circumstance Two. A number of times when driving back from work, the engine began to balk and cut out. On these occasions, it usually restarted easily, sometimes not. When this did happen, it was always within the first mile. It never lasted the duration of the trip. If I started out and got up to speed before it happened, the momentum of the car would keep the engine running so I would not have to stop. I also noticed this same behavior if I started the engine and left it idling for more than, say, three minutes before leaving. When I tried to go, it would stall out sometimes. These incidents happened when it was not raining or wet.

Circumstance Three. Most mornings, I make an early run about a mile away, then park for an hour or so for a walk. There are several stop lights on the way. Often I feel the engine intermittently hesitating or running poorly. If I stop or slow down for a light, the engine balks and takes time to "catch on". On two of these occasions, I stopped to fill up on gas about 1/4 mile from home. When I attempted to start again, it took about 10 minutes of cranking to finally get it going. On each try it seemed to almost catch on, especially after I stopped cranking. Sometimes it would start, but it would stop running as soon as I gave it some gas. This behavior does not seem directly related to water, however on some of those mornings it has been foggy and water may be getting in on the sides of the hood.

Circumstance Four. I had reason to superficially wash off the engine several times recently. The engine will not start up right after washing. If I wait about a half hour it will fire up just fine. This is definitely due to water getting in somewhere.

Water seems to be a possible explanation for the hesitation, difficulty starting, and stalling, although it would not explain the behavior on dry days. I wonder if there is something under the hood that is getting wet just from dew getting into the engine compartment and causing this. If so, I plan to cover the component that is getting wet. I realize the customary way to begin dealing with this is to change the plugs, distributor cap, and ignition wires. I did change the plugs. I need to complete other work before changing the other components. The throttle body was cleaned and did not appear to affect this problem, but it did cure a few other problems.
Ideas on what is causing this?