Unstable diesel generator output voltages causing irregular behaviour in uninterupted power supply (UPS) units
Check generator fuel filters, a blocked filter will cause the machine to fail to maintain speed when the load is applied and will cause this.
How much other load is there on the genny? Adding a few kVA of fixed load to an otherwise unloaded machine will likely help.
If the gear is rebooting due to UPS switching, then the UPS is not suitable for that equipment, some gear with very poor hold up really needs a online double conversion UPS which does not have a switching action between battery and mains, a separate issue to your genny problems, but something you may wish to look into.