Just doesn't start =[

so i bought a used 1991 buick skylark 4 cylinder 2.5L
i've been fixing it up for a while mostly the basics (tune up, brakes, shocks ect...) but when i start it it will run but after it stutters like the alternator isn't working i went and had it check but to no avail...=[

the battery dies and the lights dim like its the alternator but it tested fine so any help would be greeatly appreciated