I'm running the HKS EVC 6 and I continue to boost spike up to 17-18psi for split second, then it will drop down to 15psi (where I'm tuned to). I tried unplugging the EVC to see how it would react and it does the same thing, it will quickly peak to 13-14psi and then drop immediately to 10psi and hold it there (10psi spring) at WOT. So I assume that eliminates a possible boost leak.

Is it because I run the APS TT kit with internal waste-gates? What else could be going on?

 

how long is your boost reference lines to the wastegate? If they are really long it will take time for the 10psi to hit that wastegate diaphram meanwhile the boost is up to 13psi before the wastegate sees the boost.

usually boost will go higher in the begining and taper off as more and more goes through the exhaust