wierd issue engine playing up

[Dr HMS Derv Destroyer]

having a wierd issue, the engine randomly becomes un responsive and looking at the dials , the revs bounce up and down aroind 1500rpm. it doesnt cut out but takes a while ( 10seconds or less ) to go back to normal revs.
also the temp needle bobs up and down quick.

guessing this can be a electrical loom related issue? power drop caulsing funny running.

only happened tpday with this sunny weather

 

[MartinR]

Coolant temp sensor?

 

[Dr HMS Derv Destroyer]

that was done not long ago with a new thermo stat. its not done it since yesterday which is worrying to think it might just stop nextime without warning