not sure if this is much help mate

when the engines runnin with belt on

is everythin stayin lined up, sometimes they can buckle or move which caused me 2 go thru loads of belts til i bought a new pulley for the belt to sit on and hey presto

the bottom pulley will be hard 2 turn due to compression in the engine, and ur alternator should turn freely (try spinnin the alt pulley and listen for any whizzing noises or roughness)

again hope this is atleast a bit of help mate