I just installed a new accufab 75mm tb/plenum combo on my 2004 gt. First trip around the block went great, but as my car warmed up I began to notice that whenever I took the car out of gear the tachometer hovered at about 1500 rpm for about 4 seconds before it dropped down to 800-900 rpm to idle. I am sure someone out there knows what is going on. I am definitely not a motor head and a little lost. A little help would be great.