Gas Tappan Visualite Oven (1959) Not staying hot

Our oven will light just fine and has no problem getting gas. The problem we are having is getting and keeping the temp where we need it. We turn the oven on to 350 for example and come back a few minutes later and the flame is barely going and the oven is not to temperature. Any ideas what the problem could be? also are the parts readily available for this old of an oven? And about how much are replacement parts for this issue? Thank you very much!