whats wrong with my citroen relay 2.2 hdi?

i have a 2003 plate citroen relay lwb 2.2hdi van. have had problems with the accelarator pedal not working(electronic one), bought a new one. worked ok for a while then doesnt work, then works ok again. also have an intermitant problem with it not starting at all. sometimes when starting, the pedal will not function for 5-10 mins, there is a noticeable change to the engine sound and then it just starts to work.it has 187,000 miles on clock, no problems at all when its going properly. been into our local diesel fitter several times but always seems to work ok when its there. any ideas??