2008 m2 112 runs rough randomly

I have a 2008 M2 112 with C13 cat engine and randomly at higher speeds (66-69mph) when pulling a hill the engine acts like it is derating. A little bit like a dead cylinder or like the Jake comes partially on. It kinda starts shaking around. I can give move throttle but it just kinda lugs. I can let up and more less coast for 1/2 mile and slowly give throttle again and it is fine. It’s not throwing any lights. It never does it when going slower or in other gears besides 10th. We had all the emissions deleted and removed and the Hp has been turned up to 425.

 

Where in fuel system would I start and how do I check for plugged lines?

 

I replaced the fuel filter but did not make a difference