I changed the flapper but still have the leak some of the time. Could the new flapper be bad or the wrong type or could the water be going between the rubber gasket and the tank bottom?

Yes, new flopper can leak. When buying floppers, you have to inspect it for warps and imperfections. Buy flopper that is individually wrapped. Don't buy the cheapest flopper or flopper packaged with dozen of others. Also, make sure you are buying flopper that is designed for your toilet. Go on line with name and model number and cross check with manufacturer. Brand of toilet should be printed behind the seat and model number is stamped on the inside of tank or on the underside of the lid.

Hope that helps

Milo

Also check that the refill tube is not stuck down in the overflow pipe. Should be attached to the top of the overflow, not down in it.

Agree and also important is to check the seat of the flapper for nicks or unevenness around the rim.