Turbo boost on stock 2004 mazdaspeed is low at 4psi. Should be getting to 8-10 psi. At idle, on warm up, often get P300 code, random misfires. Once warmed up, runs great. New plugs, coils, wires. Injectors and fuel system test fine. Any thoughts most appreciated.

 

"Once warmed up, runs great" but it's only making 4 PSI? I'd be checking intercooler piping and couplers to see if there's a leak somewhere.