If you have the stock type wastegate and you don't have an aftermarket turbo most times it is because the wastegate hole is too small and not enough exhaust gas can escape there by slowing the exhaust wheel down
same thing happened to me with my old TE-44 turbo...i was running a stock 90 degree elbow and poston's downpipe with it and once i hit 25 psi in 3rd gear it would creep up to about 27 psi....one time i was on the highway and ran it in OD at 25 psi and it creeped all the way up to 28-29 psi it was due to the very small hole...needed to be enlarged and i never did it for some reason...didn't think it was a major deal to me...i was running dual nozzle alcohol injection so the octane was there at all times...but then i finally blew a headgasket so what do i know!
The "one more turn" is not allowing the puck to open far enough to waste the boost required for keeping it where you have it set. Set the rod for about a 1/8-1/4 inch stretch to the clipped pin end, then adjust boost with the chip. Or, someone can tell you how to use an adjustable valve for control. (In place of the factory wastegate solenoid.)
The puck doesn't always seat centered over the hole. If you don't account for that, you could grind away too much and have it bleeding off boost constantly. Make it bigger, just do it in the right place.