Voltage Drop if Generator on Load

hi friends,

i have 456kw perkins genset, i set it on 70% load maximum means 320kw max b4 its good but i found if load is coming 250 or 260 my voltage is coming 370 369 & if theres no load its 384volt hz im getting on load 59.9 or 60 is good but genset on load 250 kw the voltage drop like this volt A 370 voltB 378 voltC 375 how to solve plsssss inform....

Regards.. n thank you