do it for a minute at a time and give it a good stir, or better still use a small whisk, in between. you'll find after one or two minutes, there will be a thick residue at the bottom which you need to stir or whisk to break it up into the rest of the mixture. just keep on doing this until it thickens.
i make all sauces in the microwave, never have a problem and they always turn out perfect.
Ethandron, I make custard just the same as you do! never a problem and it always turns out just right with the accasional stir. I love custard and wouldnt dream of cooking it in saucepans that have to be washed up x
I always burn ready made custard in the microwave. Make it yourself with milk, sugar and custard powder, two mins in microwave, stir, back in, when it starts to 'rise' its done . mmmmmmmm