V70 Xc 1998: runs smooth, but I'll reach a red light, stop, and then when I need to accelerate the RPM meter jumps to 2 and the car takes a while to start moving just starts to rev for maybe half a second to a second. There is also the flashing arrow indicating a fault. However I don't know what to do. Is there no transmission filter? Where is to change it and the dipstick shows black and dorty fluid. What should I do