I have a CAT 320 that suffers from power loss under load, as soon as it tries to dig it sounds like its suffocation and if you keep pushing it will eventually stall.If you let it recover, you will see a puff of blue smoke and it will work fine until you try digging again.
Check the air and fuel filters, when you change the fuel filters take the opportunity to blow the suction line of the tank, it can be full of garbage.
If that does not help continue with checking the supply and fuel pressure and if there is air in the return line.
