Sometimes it cleans, sometimes it doesn't?

Hi there,
Is anyone able to give me any pointers as I don't want to call out a repair man as mostly my dishwasher works fine. However every so often it doesn't clean the dishes and there is still lots of food on them (particularly the bottom shelf). It may well work ok the next time I run it or may not work for several times before it starts working again. I've checked the filters and it's not anything like that and I've had a new pipe fitted a few months ago so no blockages there I'm sure. I don't want to pay someone to take a look if I can't replicate the problem when they come and then they can't work out the problem and I have to pay call out charge for nothing (I'm a single mum on a very tight budget). Does anyone have any clues at all....?
Very many thanks in advance...got my fingers crossed!!!