My LS400 is not running like a V8 should. For the first few years i've had the car it ran great. I use to fool around and drive fast, and when i hit the gas it picked up speed and roared like a V8 should, now it doesn't pick up the speed nor rev up like it used to. What could be the problem? What do i need to fix?

Any lights on the dash kit up? How many miles on the car? Change any parts lately? No check engine light. 134590 miles. And a few months ago I changed out the alternator, PS pump and line because I have multiple ps leaks. A week ago I changed out the gasket valve covers. Today I changed the air filter. And about six months ago I had a tune up done. Didn't change the distributor cap or spark plug wires. I'm going to do those soon as well as a throttle body cleaning.