Some good reference information:
https://www.surpluscenter.com/Tech-Help/Tech-Help-Home/https://www.surpluscenter.com/Tech-Help/Hydraulics/Calculators/Pump-Disp-HP-GPM-RPM/https://www.surpluscenter.com/Tech-Help/Hydraulics/Hose-Sizing/If you are running a lawn mower style gas engine, it is probally running at 3,600 rpm. 1.31 in^3/rev *3600 = 20.42 gpm. If most your hoses are JIC 8, they are probally 1/2" hoses. Using the recommended flow valocities on suprlus center in the links above,
Q (flow in gpm) = 2.45 * D^2 * V V=velocity, D= diameter, Q = flow in gpm. recomended flow is 11-18 in pressure lines, 2-4 in suction, and 4-13 in return lines (from surplus center site above)
I don't know what size lines you have but say your hoses are .5"
V=Q/(2.45*D^2) so V=20.42/(2.45*.5^2) = 33.3 ft/second. The 2.45 accounts for all the unit conversions. That is double the recommended velocity, if you mix in a 3/8 fitting somewhere in the line and you have 59 ft/s. If you go to 5/8 V drops to 21, 3/4 hose and you get 14, 1" hose, 8.3 ft/sec., 1.5" hose = 3.7 ft/second.
This means that you need a 1.5" hose from your tank to your pump, 3/4" lines from the pump to your flow divider (and a divider rated at over 20 gpm), 1/2 in lines to your valves (flow is split in two), then 1" or larger where everything comes back together to go through the filter and tank. And of course the filter needs to be sized for at least 20-30 gpm.
I would start with lookin at all your hose sizes, and thinking about the velocities (that in turn generate back pressures of 400 and 700 psi) before changing any more expensive parts. If all that is fine then something is piped wrong. They could also have a valve on there that is rated for 5 gpm and your feeding it 10, it would still work, but not without causing heating issues.
Before you start replacing hoses, you need to complete the pressure measurements so that you can confirm your issues
Again.............please post pics of your machine and hose routing or we are just guessing and not really helping.
