The auto gearbox on my 1994 Volvo 850 20v GLT gets lazy when it's been
standing for a few days; it doesn't want to shift up when I drive away,
though when I temporarily move the gear lever to the "3" position the
box shifts up as normal, and when I then change back to the normal Drive
position, the problem stays gone, regardless of how briefly it was in
"3" for.  If I don't do anything and just keep driving, eventually, the
box does begin to work normally.

I have checked the oil level carefully as per my Haynes service manual
instructions. Is it worth changing the oiil (although the manual says I
shouldn't need to unless it is particularly burnt)? As putting it in "3"
solves the problem, isn't this more likely to be some kind of
electronics issue?

Many thanks for any advice.