1997 Renault Scenic Engine losses revs and cuts out intermi

Engine Mechanical problem
1997 Renault Scenic Manual 113K miles
----------------------------------------------------------------
Hi,

While driving normally the engine sometimes loses revs and judders and jolts. Sometimes it is bad enough to stall the engine. We have changed the HT leads and put engine flush through the fuel system.
Have you any ideas as to what the problem could be?
It doesnt do it all the time, it can run fine all week to work and back (about 10 miles each way) but when driving a bit more at the weekends it will play up.
Any ideas??

Many thanks
Sarah Sandy