citroen relay keeps cutting out?

we can get to a certain speed or gear change and then just dies on us,e were told to change thermostat and sender unit plus oil filter,still same problem,we can be going along normaly,engine is as sound as a pound,then all of a sudden just loses power,like i mentioned changed the parts suggested and still having the problem,seems to be when getting in to 4th and 5th gear,any advice would be greatly accepted,as a small business and on a budget,just cant afford hefty mechanic bills,thanks,Nigel