2011 D13 loss of power at 1,600 RPM

My 2011 Volvo D13 is losing power at 1,500 rpm about 70mph going uphill, downhill or on bumpy roads. The speedometer needle jumps back and forth between 60mph and 70mph. I recently replaced the housing primer pump and a gas line connected to the engine, since it wouldn’t crank up we thought this had fix problem, but the problem is still there.

 

The reason why the speedometer bounces back and forth is because it losses power. We thought it was the fuel water separator filter and got it replaced. while getting exchange at TA the truck didn’t start back up. We had to towed to a mechanic who found the house primer pump leaking along with a diesel line. We exchange both but the problem persist.