A couple of points to address:
1: I'm glad I'm not the only one, always reassuring.
2: Re: John's point, I have only noticed it in the hot weather, particularly (I believe) after replacing my thermostat so that the engine actually holds at operating temp (92°C).
3: This vehicle is my intro to automagics AND to old Volvos. Given I've only had the 240 a handful of weeks, it's hard to say what's normal. I bought it with the throttle cable tightened to the point of having 'around town cruise control' as it never quite shut, but I'm gradually bringing it up to a solid stage 0.
Most likely I'll just wait until autumn when it cools off a bit and observe the behaviour then... Any reason for the hot weather magnifying the effect?