2006 Mercury Marquis

Replaced the battery in March. Had battery checked and was told I needed a new one and that fixed the problems I had a the time. Last night driving home about midnight with lights and ac on the traction light came on and the lights dimed. We were about 6 miles from home and kept driving. A couple of miles from home the dash lights went out and came back on, the went out again along with my head lights. Whe lights would stay off for say 10 seconds then come back on and then off again. This kept happening until we got home. I guess it may be the battery or the alternator. I checked the battery terminals for loosness or corssion and the belt for looseness and seems to OK.