Pics?

I recently got a lightly-used Schwinn that has an adjustable stem. I wanted to raise its angle a bit, and this stem needs to be disassembled to change anything -- which I don't mind, because I'm pretty confident that it won't slip, either.

There are also several spacers on the steerer tube that I could rearrange to further adjust the height. Take off the stem, take off a spacer or two, put the stem back on, and stack the spacer(s) on top.

If this doesn't help (and it might not), you can always swap the stem out for another one.