hard crack candy coming out too sticky

Get another thermometer and check that your thermometer is calibrated correctly. Many of the candy thermometers from China are ± 20 C

I have the answer to this and as someone above said, it is to do with humidity. I make toffee apples to sell and have found out by trial and error that

1. you have to make sure they are definitely heated to 150°C so that the mixture goes immediately solid when dropped in cold water and
2. Wrap them in cellophane the minute they are cool enough, so probably within 5 minutes of cooking. They get sticky quickly if left in contact with the air. If you wrap them they will remain hard like glass and not tacky.

Duplicating RBerteig's comment as an answer for posterity:

Humidity is the enemy of hard candy. It will turn it into hard, sticky, goo. I'd try sealing a few pieces in an air-tight container along with some dry rice (or better, silica gel packets) and see if that works better.

Instead of powdered sugar, try dusting the candies with a tiny bit of cornstarch. This has the advantage of not turning sticky itself.

Old thread, I know, but for anyone else looking for answers - make sure your thermometer is not touching the bottom of the pot. This can throw off even a well-calibrated candy thermometer.

Your recipe may be the problem. The all sugar with water and cream of tartar tends to have the problem you have stated. I prefer the 2C sugar, 2/3C corn syrup, 3/4C water, flavor/color to desire

Never freeze or refrigerate hard candy, if a fan such as your stove fan is going turn it off or move the cooling candy out of the way. Be sure to melt not just dissolve the sugar slowly in the first stage of cooking when you are stirring, and do NOT stir once you enter the boil stage. Flavor and color I add after I have removed from heat and boiling has started, stior very smoothly outward spiral then inward a few times.

Cream of tartar softens the candy, corn syrup will prevent it going "sticky" at room temperature so easily. While powdered sugar will help I find it isn't necessary with the corn syrup version, though humidity and temperature likely play a role in this.

It will probably still stick to parchment paper but I have had no sticking to other pieces when just thrown into a plastic bowl.

I ran into this problem too: I didn't cook all the water out of my sugar corn syrup mix. I let it sit and simmer for a few hours before it would finally set. That was very time consuming but came out great.