Anti-pollution fault... with icon light on?

Hi guys, I have a on-going problem with my 307 hdi 2.0
desiel. An icon used to appear on the dash with a beep also, which sounded faint like the battery was dying, plus anti-pollution fault would appear.

Ive bought it to a garage twice for this problem, as the power of the car failed when the light was on, and the only way to go faster would be to change gear on low revs.

Now it has been fixed or so I thought.. Now when I drive the car its fine, but when i go above 70mph thats when the light comes back on and the car loses power. What ive also noticed is that if i diconnect the battery and re-connect it the light goes off and the car is fine again, until i go above 70mph.

CAN ANYONE HELP ME PLEASE?/