Fueling Problems?

[mycerbera]

Everytime I go to a certain Shell garage round my way, and after filling to the brim my tank with Optimax, I start her up pull away then stop at the junction (which is on a very slight upwards slope) and she cuts out on me. Then really struggles to restart! Engines turning but not firing! Finally does, then next time I slow at a junction same again! Cuts out and struggles to restart. Any ideas as to what this could be?