2001 DS struggles/wont fire

[Krambbor]

Hi all,
I have recently purchased a 2001 DS and have been having a few issues.
when I first get on the course the cart seems to run fine, but as the game goes on it struggles to fire. the starter is turning but takes a while for the motor to fire, once running it seems to be fine. last game after 16 holes, it gave up, starter turned freely had a few coughs but would not fire.
when I got home last night had a quick look cleaned the spark plug and eventually fired and ran again.
So, I have checked the starter, rev limiter, the spark plug is new but keeps fouling, new filters and oil, has spark, good fuel flow from pump, solenoid and voltage regulator tests seem fine. I have also adjusted the throttle cable after reading previous threads so the micro switch is clicking prior to the cable loading up.
I am using a NGK BP5ES spark plug and after some research last night was thinking of trying a BP4ES for a hotter plug. would this work?