Pc35-mr3 sn/16776 Machine operates fine when it's cold as the hydraulic temperature rises with first 10 minutes of use the boom lift function starts to fade. Lifting at full extension of the stick becomes impossible, curl stick in closer machine will boom up slowly taking 15 seconds to reach full lift. Extended stick out still with load boom holds, stick moves at a normal pace as temperature rises. I'm thinking it's a control valve issue but I'm not sure of what. Is there any adjustment to pressure relief valves or the pilot pressure?
Could be bad piston seals in the cylinder, or a control valve issue. Sounds like your other functions work normally? So unlikely a pump issue. Do you have a hydraulic pressure test kit? This one is pretty cheap and has served me well https://a.co/d/17UUAS8 . You’ll probably need to get some NPT to ORFS adapters (my pc35mr-2 used ORFS, dunno about the -3). Pull a line off the cylinder and connect the gauge, see what kind of pressures you get when hot and cold.
Have the same machine and had same issue, with tracks getting weaker the hotter the machine got from use. Rebuilt center swivel to correct issue with tracks and blade. Boom still got weak like you describe. Packing was replaced not long ago so I pulled and resealed some spools in the main control valve, most were in poor condition.