I have an 03 turbo diesel van and it won't start?

If I spray cold start into the air filter it will start and run quite well althogh I feel it is lackking some power especially when pulling off from junctions and maybe climbing hills.
It has had a new air filter and heater plugs.
It started doing this about 3 months ago by not starting when the engine was warm and after a few weeks from cold as well and thats when I had a new air filter and heater plugs which sorted things for two weeks and now having to use the cold start all the time.