power loss and cutting out

Help.
I have just got my barn find Defender through an MOT and want to start using it but when the engine gets near the top of the power range it loses all power as if it has been turned off.If you take your foot off the throttle most times it will carry on running but sometimes it cuts out altogether.I tried to make it do this when stationary and the revs died and then came back several times as if I was gunning the engine.
After the engine has died you have to turn the ignition right off before it will start again.
All suggestions greatly appreciated.

Easy stuff first is the stop solenoid wiring terminal good and tight on the solenoid.
Have you changed all the filters?
My S2 came from a farm and the tank mesh was full of grass and other crap.


Lynall