2000 Chevrolet Blazer Serpentine belt squeals

I have a Blazer 2000, 4x2, Vortex 4.3l. I went to the mechanic to change some hoses and general service. While still in the mechanic's hands, the belt started to squeal loudly. The mechanic changed the tensioner pulley but the sound didnt disappear. The mechanic said it was the harmonic balancer pulley. I bought one in autozone. The sound disappeared for 3 days and started again. The mechanic said it was the bad quality of the harmonic balance pulley. I bought a second H. Balancer pulley in autozone, different brand than the first one. The mechanic said it didnt fit. We went to a chevrolet dealer parts department, the mechanic compared their HB pulley to mine and said it would not fit because of some minor differences. I bought a third one in autozone (different brand than the previous two). It didnt fit but sent it to lathe to make it fit. The sound disappeared some hours.

I dont know what to do. It seems the solutions last a very short time. I dont think the harmonic balancer pulleys of autozone are so low quality to last just hours. It seems that after some hours of work, the harmonic balancer pulley, or another pulley, misaligns. The wobbling of the harmonic Balancer pulley is almost imperceptible. I dont think it is the serpentine belt because at least some hours works ok. ¿What can I do. Thanks so much.