2002 Renault Clio car keeps slowing whilst in drive

2002 Renault Clio Manual 53000 miles

hi i have no idea about cars sorry but im having a problem with my clio. whilst driving my car all of a sudden it slows right down and i can not go faster then 20mph approx, a warning light on the dashboard appears which can be seen on the picture uploaded located underneath the petrol light on the dashboard. i then have to pull over and switch the engine off and wait a few minutes until the light disappears. however in one car journey this can happen several times. this happened a year ago and when i took it to a mechanic he did not find anything wrong. please help, have you any ideas what is causing this?