RobC4sX51

Today I had an unusual experience with my car. Let me first say it does not have PSE but instead has a SharkWerks side muffler delete mod. I also have an IPD plenum and a FVD tune. And all of these have been in place for months or even over a year for the IPD & SWs. Also, I have just under 21k miles on my car. So here is what it did: After it was warmed up I max accelerated in sport & auto mode until it shifted into 3rd. No issue, didn't hear anything or feel anything different until I slowed down and went to sport mode off (normal mode). The car felt like the brakes were slightly on! Very sluggish and uncharacteristic like performance. I drove it for about 4 miles and even tried sport & sport plus mode and it still felt "wrong". I shut it down, restarted it and it was the same! I drive home, parked it, and then after about 1.5 hrs went back out & started it. ALL NORMAL again! Drove around for a few miles and it was back to great again. I'm thinking that my center muffler vacuum operated valves got stuck open? And so at slower engine speeds it had less back pressure and thus the performance was reduced? But this doesn't make sense as PSE cars can open these valves at any time/engine speed! So What else can it be! Thx.