Sustained spells should NOT require drain resistance each round - that goes against the whole concept of Sustained and Permanent spells (which also have to be sustained for a while). Instead of more drain, these spells require concentration by the mage to keep the flow going.
It is in certain ways much like a light bulb. A light bulb will most likely burn out right when you flick it on, becasue at that point there is a beginning wave of electricity going into it. Once it has passed the crest, the lightbulb has a much lesser chance of blowing.
The same goes for a mage - He looks at his target and synchs his aura to the targets. He then coalesques the energy and sends it traveling to the target through the astral plane where it channels into the target through the link the mage made by synching his aura. Once this is done, the mage only has to concentrate to keep the energy flowing into the target, he does not have to take the enegry into himself, but rather, he holds onto to the remote the remote control and fiddles a little bit with the energy, but thats all.
A mage SHOULD do damage every phase he has an action. Since he can change his focus to another target, there by doing damage, he can therefore damage the person again.
Think of it like this. The mage has the above remote control to the spell, when his action comes around - he can treak it again, thereby doing more damage. No - he should not have to roll successes again, the spell is already grounded, though you might want to roll and see if he keeps the spell on target if it is a damaging manipulation (which are like guns in that they travel the distace).