1983 automatic SL280 - Won't go through the gears

[johncw]

Doesn’t happen all the time, but sometimes (normally when starting from cold) the car gets to a certain point and then doesn’t want to go up any more gears, you can tell it’s trying as there is a kind of jumping effect. I can normally get around it by taking my foot off the accelerator then trying again so it’s not a major problem yet, but it would be good to know what the cause is - can anyone help?