2004 Chevrolet Impala Shifting and speedometer problems

Hello,
I own an impala and recently it has started to have problems. It does not always occur, but sometimes the speedometer stops working and the gears will not shift. I got it at around 80 000 miles and it was working fine, but as I use it more and more, the problem occurs more often. It did not bother me too much because sometimes it went back to normal after a few minutes, but last night I was driving on the autoroute and could not drive more than 25 m/h since I was in first gear and couldn't change. The speedometer problem is not very important to me, all it does is that it just shows as if I'm going 0 m/h. Those 2 problems always occur at the same time, so I'm not sure if it is a transmission problem or an electric problem. I'm not really good at mechanics, but I think that if it is a transmission problem I could be able to do it by myself, but an electric problem would be more complex. Any help about what can cause this problem is greatly appreciated.