raplaced alternetor now battery light comes on and off about every 5 seconds on 2002 Chevrolet Silverado 2500 HD

checked battery voltage engine of (13 VDC)
checked while engine running (15 VDC)
Battery load tested at local parts store (OK)
the only major work done recently was replacment of transmission.
Prior to Alternator going bad had and continue to have i issues with ignition in that I get all the diagnoctic dings and lights on dash-Turn key and nothing-no cranking no nothing- remove starter relay and put back in starts right up- already replaced relay but still does the same thing intermittenly. All out of ideas

Asked by for the 2002 Chevrolet Silverado 2500 HD