Issue with Nissan Micra losing power in 4th/5th gear. ?

I have a 1998 Nissan Micra, 1.0L, 90,000 miles. It has been working fine but recently it has began losing drive at about 40mph. There is no acceleration and there seems to be a slight burning rubber/plastic smell. The car drives fine for first 10 minutes of a journey then this issue kicks in. Any idea guys?