My LC runs very well almost all of the time. Intermittently when I accelerate, It feels like the acceleration is in slow motion. If I push the gas pedal all the way down, it will do a hard downshift into the lower gear. When I let back off....... back to slow motion, almost like trying to suck ice cream through a straw. The engine idles great, in neutral sounds awesome at normal rpm, no misses or hesitation I have replaced the fuel filter, spark plugs and the cap and rotor are fairly new. Any ideas what may be causing this.