Automatic trans issue

I have a mate with a problem with the auto trans in an 80 series.

When it is powered it up the gears the changes are as expected. However sometimes when it is say more or less idling around the streets, the change from first to second is fine but the change from second to third is quite clunky as if there is nothing then a quick grab.

This occurs under 1000 rpm. An example is from a stop sign to a turn into a downhill where speed does not get above 20kph to the next turn right or left.

When it is gunned, its changes are quite smooth all through the rev range - up and down. So this leaves my mate to manually change to second where he thinks that this may occur again eg on suburban streets.

So my learned friends, any input??

Thanks in advance.