Rob Bradley wrote :

I'm having a bit of a nightmare with yogurt - sometimes it curdles,
sometimes not. Can you help ???
...... Often this works fine,
but occasionally the yogurt curdles after 5 minutes. Do you have any
idea what the problem could be? Is it temperature or the other
ingredients ?

I can think of 2 possiblities.

1)
You don't say what kind of yoghurt you are using but I would bet good money
you are using low fat. Low fat yoghurt will tend to split (curdle) when
boiled. The answer is to use full fat yoghurt also known in the UK as
"Greek-style" yoghurt. The taste is far better too. 

If you cannot get full fat yoghurt then you will need to add the
yoghurt (having warmed in gently) at the end of the cooking and NOT
boil the curry again. The yogurt will have a "raw" sort of taste
but........if that's all you've got. 

Another option is to substitute sour cream for the yoghurt but only use 
half the amount

2) The other explanation is that you have added something acidic
like lemon juice or vinegar. That will make even full fat yoghurt split if
the acidity is high enough.

The same advice goes for cooking with cream.