Are you cooking side ribs or back ribs? Side ribs tend to take a little longer.
Personally I cook mine at closer to 250°F and have not noticed any adverse effects. I don't foil mine either, but that just personal preference.
When checking your ribs look for the meat sliding up the bones, that's your first indication they're nearing completion.
Some use a bend test. Using tongs, pick a rack up in the middle of the rack. If the meat bends and tears away from the middle bones, you're almost done as well.
You can try pulling on a couple of center bones as well, if the meat tears fairly easily you're good to go.
My timing usually goes something like:
Back ribs 4ish hours.
Side ribs 4-5ish hours.
These are guidelines only, experience is your best bet.