Lucas pump occasional high rev problem?

[Mitch]

hey guys i'm new to the forum, (have read a lot on here before coming a member). Basically every now and then my car seems to what i can describe as stop delivering fuel at around 4100-4300rpm, when i hit that amount of revs it seems to just 'drop off' and not want to rev anymore/higher, but other times it will rev to 4900-5000rpm without the hesitation, any ideas? i know the Lucas isn't the best for high revving but surely if it was cutting the fuel at 4k ish it would always happen in every gear? not just when it feels like it? Thanks! Mitch.