Ah, yes. I've learned to expect it, though- any major/boss Trainer battle includes items, ie Full Restore (or the Potion line, depending where you are in the game). It's just the way they go. However, I've noticed that the Trainer only uses one item on a Pokemon- for example, Cynthia's Togekiss is in Red HP. You choose to attack again, and naturally there's a short pause, a sound effect, and then that terrible text: "Champion Cynthia used Full Restore!".
The attack you selected brings Togekiss back down to red. And that's it- you attack again, bye-bye Togekiss. I suppose this'd have to happen, else some Trainer's would be impossible to beat. There's also a limit to the amount of items the Trainers can use in a battle (which is also a factor in the difficulty of a Trainer)- in the GBA games it's 4, anyway, but I'm not sure about Gen IV games.
It doesn't really get me anymore, I get the health to about middle-yellow (ie, leaving just enough for the game to think it's safe). Then I go all out, and bam- win:)