this sounds like a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.
If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,
Ideally measure the pressure at the injectors ( if fitted some were on a carburettor) and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.
Thanks for the reply. I will have a look at this and get back to you if there is anything more I need to know. Sorry it's taken me so long to look at it have been away.