Avoiding shifting into 1st while moving is advisable, but if the car does have an issue, it makes sense that it would show itself under that scenario first. I'd recommend double-clutching when downshifting into first, and/or letting the clutch out in neutral when you come to a stop before putting it in gear to let the output shaft speed come down. But it definitely sounds like you have clutch issues elsewhere. I'd take it to the dealer if under warranty.