Renault Kangoo rpm dropping

My rpm keeps dropping to zero when I take my foot off the acelerator to slow down or stop ( it restarts with no problem at all ) have had this problem on and off for years .When it first started my garage put in a new speed sensor which seemed to fix the problem for quite a while, ( about 1 year ) then it started to do it again, this time when a new speed sensor was fitted it didn't make any difference. My garage has tried numerous things but nothing seems to work. Its very difficult to pin down the problem as it can go weeks without doing it or it can do it every day for a number of days. The car has been on the diagnostic machine but it doesn't show any problem.Please help me.