Volvo S80 2004 jumps when it's about to stop or Move!

2004 Volvo S80 Automatic 40000 miles

Hi.
I bought a used volvo S80 model 2004. This is my third week.
I noticed that there's an issue I'm worried about.
When the car is left not driven for long time (for example 12 hours) and then you drive it, you can't feel any problem with it.
But after a while, when you want to move the gear from 'N' to 'D' (when signal goes green), the car takes about 2-3 seconds to respond.
You can feel it's doing a jump!
The same thing happens when you're about to stop the car (while reducing the speed & before it stops). It jumps only once.
I'm afraid it's a gear problem. I asked about its price & I knew it's very expensive.
I was surprised to know that I can't change the oil of the gear(Transmission) or even the filter.
Please, if there's a need for that & that there's way to do so, I hope you give me a hand.
I really need your kind help and assistance.
By the way, I'm living in Dubai - United Arab Emirates. If the car needs any spare part (hope not), I'll try to buy it online in case it's cheaper.

Thanks in advance :)