Check the DISA.
From a friend: It opens and closes a flap in the intake manifold to give you more torque in the low RPM range and more power in the upper RPM.
At <3750 RPM a solenoid is energized which controls the vacuum to close the flap, it allow air to enter thru one tube but at a higher velocity for more torque output.
Above 4100 RPM the solenoid closes, the flap opens and air now enters thru 2 tubes increasing the volume needed for high RPM and power output.
I had similar symptoms as you when mine went.