What can I do to fix my car

Bought used car about 2 months ago. Car was running good service engine soon light came on code said gas cap replaced gas cap. Soon after service light came on battery died hard starting sometimes start sometimes cranks like dead battery. Had charging system checked they said alternator putting out low voltage. Replaced alternator and serpentine belt no help. Matter of fact old alternator puts out more voltage then alternator in car now. Service ligt comes on security light comes on tachometer jumps up and down fuel light comes on and temperature guage drops down. Then most lights go back to normal except service ligght and security light are consistent please help.