CM870 Jake problem

[DKenworth]

Hello guys! I have Cummins CM870 with 1.238xxx miles and my engine brakes works weird. I did change 2 solenoids and also new wires that runs under head cover, tune up is done too, I did it all last summer and nothing helped. I noticed that when engine is cold, under 180F Jake brake works as it should but after it reaches 180F its become weak and shakes whole truck when is on. But sometimes when I go down the hill I turn Jake's on and at beginning it shakes and after a while something may kicks in and jakes works fine...when Jake's are weak I'm getting only like 3-5psi of boost, but when Jakes strong with 16RPM boost goes up to15-18PSI. Thank you for your help.