I am not the native speaker but look:

http://www.google.pl/search?hl=pl&q=add sugar gradually&lr=

I've seen it like this.

I think it fits for a cooking recipe

this is what I use.

Using a wooden spoon, cream together the butter/margarine and sugar until it is light and fluffy. **Add the beaten eggs and vanilla extract a little at a time**, beating well in between each addition – http://www.treehuggermums.co.uk/articles/parenting/article.p...

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Note added at 2 hrs (2008-07-16 16:52:06 GMT)
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Many more examples if you do a Google search.

Boil for one hour, adding sugar a little at a time and stir till dissolved. Only then bring to the boil again and boil for another hour or until it jellies on a cold saucer.
http://www.puddington-village.net/recipes/recipes_1.html

Add the stock a little at a time mixing thoroughly to prevent lumps then add the tomato concasse, bring to the boil and skin.
http://www.graigfarm.co.uk/welsh_mountain_mutton_recipes.htm...


Add dry ice **a little at a time** until the ice cream has absorbed it all.
http://www.independent.co.uk/life-style/gadgets-and-tech/fea...

Add the sugar,** a little at a time**, whisking the eggs constantly, until all the sugar has been used up and the eggs are white, stiff and glossy.
http://www.bbc.co.uk/food/get_cooking/recipes/022.shtml

Might be an alternative, though I think that GRADUALLY is the best option, at least what I have seen here in the UK in most of the recipes I have ever used.

I think you could be literal too and use 'at intervals' but only if the recipe calls for precision about this (you have to be really careful sometimes, like when you fold beaten whites to make a soufflé or mousse)

Collins Sp-En

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Note added at 16 hrs (2008-07-17 07:34:31 GMT)
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OR

occasionally