Sounds like it's 'hunting', which can have a few causes. Gummed up carby emulsion tube holes is a common one.

A couple of good past threads on this topic:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=64380

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=38417



Cheers,
Gadge

"ODK Mods can explain it to you, but they can't understand it for you..."

"Crazy can be medicated, ignorance can be educated - but there is no cure for stupid..."