I would start with a factory reset on at least one of the meory banks. the manual will tell you how.
then set mode to P, A or S...not M then set metering to matrix ensure bracketing, exposure lock etc all turned off.
Take a few shots at different zoom settings. If the exposure still varies then there could be an issue with the new lens. Mounting other lenses, followed by some test shots, will tell you if it is a lens or camera problem.