Good hot and cold water pressure at faucet but when switched to shower, temperature of water drops drastically (minimal hot water coming through) and water pressure is very low. Shower head has been switched, mesh has been cleaned of any debris, and black rubber ring has been changed. What can I do?

Good hot and cold water pressure at faucet but when switched to shower, temperature of water drops drasticall By "faucet" do you mean the tub spout or do you mean the lavatory faucet?
What type of valve do you have? One handle or two knobs? Is the diverter on the spout or in the valve? What's the brand name? More details please. Tom