Terex TC 35 Drive motors

Hi, I have a 2007 TC 35 that is having issues with both side drive motors and it is intermittent (75% of the time) all of the boom, blade, bucket, and thumb functions are working fine, not slow or lacking any power, but when driving it seems like there is a lot less power going to the tracks. Its not stuck in low speed mode, i have verified that the hi-lo speed switch does work, and the difference in speed is huge between the hi-lo. The problem is more the low end torque, pulling itself up steeper terrain it is lacking forsure, but it does reach top speed on flat ground ill add some backstory, added a thumb attachment aftermarket when i got it, keeping in mind this was my first real piece of equipment i just installed it and began using it, i realized that the thumb cylinder was strong enough to start bending the metal for the thumb itself, i then learned about pressure relief valves and how to properly set them, so once i did that the thumb would no longer crush itself. Now having said that, when i am driving with the tracks i am hearing the same kind of pressure relief sound when the tracks seem to be not putting down enough power, it definitely is a by-pass like sound. i have followed my drive motor lines back to the distribution block and it looks like the drive motors do not have the Allan-Key relief valve adjustment, just connects to the block and thats it. i have verified my hydraulic fluid is at the correct level, the filters have been changed, i didnt find any brass or metal inside the filter element. things i have not done yet are check the pressure at the drive motors, i also havent personally checked the fluid inside the final drives but when i bought it from a friend i was told they were serviced and you could see there plugs had recently been pulled out with some oil residue behind. im going to assume because both sides are being affected thats its not a problem with the drive motor itself but something up the line. I need to pick up a service manual, hydraulic pressure gauge and test whip. I realize there is probably a lot of possibilities that could be causing this, but i figured i would take a shot in the dark to see if anyone here had any ideas before i get back out to the unit. Thanks