Popped out to the local petrol station to fill some cans up earlier......pulled out of a junction got about 100yards down the road, engine cuts out

battery light comes on then restarts its self.
*Hmmm wierd i think to myself. Normally i`d put it down to to the rain but I did a 200 odd mile to F.I.T.P last sunday in the pissing rain

*
Get cans filled up, drive back home......bloody thing cuts out again....this time doesnt restart and I coast to a stop. Great the one time I nip out and dont take my mobile with me

Turn the key and she restarts

limps back home incase it does it again.
Got it home, battery was looking abit worst for wear so changed that and cleaned all the cables/earth points. Take it for a spin.....same thing. It only seems to do it under load when you floor it. Checked out the starter motor cables they`re all tight and in good condition. Checked all the plugs none are loose.
Anyone else had this problem?