Long story short, cold start... Boost is good, hits around 22 or 23 psi. After driving for around 20 minutes, boost only hits a maximum of 18 or 19 psi.

Mods are:
Cobb SF Intake
AMS UICP
COBB AP, stage 1 with SF
Catback exhaust.

Recently just replaced the radiator cap (due to coolant rising in overflow tank), replaced spark plugs, did an oil change, and replaced the timing chain. Also have replaced the fuel pump relay. Car is at around 79,000 miles. A bit concerned that it may be Exhaust Manifold, but maybe a boost look? Just not sure why it starts off boosting fine then lowers.

 

Mine does something very similar but haven't figured out if its pulling timing or generating less boost as I don't have a boost gauge. I do have some slight exhaust staining on the heatshield from right around the wastegate. Are the X's known to have weak bypass valves to let off boost as they age? My car is basically stock. RRE tune, modified factory muffler, 3port, AEM air filter. 156k mi.
Jason

 

Sounds like a heat soak issue? Maybe asking a bit of the stock FMIC?

Just a thought