Peugeot 206, low rev count and wont drive above 30mph...Help?

My Peugeot 206 xsi 1.6 2001 will not accelerate above 30mph and the rev count is very low when standing. I have had a few people look at it and they can not find the fault. I have changed the throtle body and numerous other parts as recommended by different people but non of this has fixed the problem. Really do not want to have to take it to Peugeot as will cost a lot just to find the fault. Can anyone help?