I have a sudden stutter/drop in power most noticeably in WOT around 4k and 5k in 2nd and 3rd gear when pulling hard in my Evo8. It seems to make a quick pffft sound and I temporarily and briefly loose power but it regains quickly where it left off. I have had this symptom once before a year ago but a boost leak test up to 30PSI (I run at 28 peak currently) came up with nothing. I later found an EGR related vacuum hose had become disconnected behind the intake and once I reconnected it, the problem seemed to go away...but now it is back. In the following diagram I have highlighted the previous hose that came loose but I have checked everything I can feel back there and I don't find any loose connections currently. Any ideas on what to check and how to diagnose this?
- Plan to do a boost leak check again just to be safe but I don't think that will reveal anything EGR vacuum related.
- Any way to test if this could be a vacuum hose related to EGR?
- Any other ideas?

 

Maybe do a boost leak test to start off, then take it from there. If you are handy under the hood, you should know where all the little vacuum hoses are, just run over all of them and listen for a leak