2005 Chrysler PT Cruiser 20 Mph wobble

Check for a bent rim. Sometimes you'll feel a wobble at low speeds, but when the wheels turn faster, it'll go away, sometimes it won't.

Also, a lower control bushing will sometimes cause this problem.

When you hit a bump, does it feel like the car drifts a little?

This is a hard question to answer without conducting a drive test. But those are two possible causes.