Mustafa sattar

Hi I have Audi A4 B6 2002 2.0L was running very well but I was going for fun little bit far i mean distances from my place to that place was more then 150KM when I done 100km continue drive car start pushing like some one hitting from back side and I feel gear automatically changing and I saw dashboard there was lights showing PRND was blinking I stop car and check water oil every thing was perfect after half hour I again start car drive well but now when in morning I start car and drive after 3to 5 KM distance drive car start same thing like some one hitting from back and PRND LIGHTS START flashing I have to stop car for couple of minutes and then start so car running again very well I don't know happens with this thing so plz if any one know this problem solution plz help me my car petrol average is very good every thing is perfect but only I m facing this problem should I do transmission service like filter and oil change or any thing else