I drive a Subaru Legacy Turbocharged GT 2008 model with an EJ20 engine.
When I press hard on the acceleration to boost the tachometer upon reaching 4-5 thousand RPM's it starts jerking and there is no acceleration. However, if I accelerate slowly I don't experience any issue at all. The fuel filter and pump were checked and are OK, so are the plugs. An engine diagnosis was done and revealed nothing.
What could be the issue?