Does the boost rise smoothly over a reasonable rpm range, or does it just kick in from ~0 to (over) maximum pressure?
If it rises spontaneously check your wastegate if it's stuck or always slightly opened. Also check your turbo for shaft play and free blades - wouldn't be the first turbo with a nut or bolt in it...
Check the control lines from/to the WG solenoid. Not sure if it's identical on the older ECUs (please someone verify this!), but with the later evo ECUs the valve should be open between pressure side and wastegate actuator on ignition off (or solenoid unplugged) - the ECU closes the valve (100% duty cycle) at ign. ON (or from ~1500rpm upwards) and decreases duty cycle to increase pressure on the wastegate actuator and open the wastegate.
Its a failsafe configuration - if your solenoid or electric wiring is faulty, you'll only get "wastegate pressure" as the actuator will be directly connected to pressure side.
If its connected the wrong way your ECU will pass through the pressure to the wastegate from lowest RPM until the WGDC decreases - boost will increase slowly and at higher rpms it will completely overboost...
Also pressurize your piping from right after the turbo outlet to find any boost leak. Disconnect the hose at BOV outlet and keep an eye on it (block it with your hand) while increasing the pressure. None of the >20 year old galant BOVs i've seen so far were able to keep more than ~0,7-0,8 bar. Most of them started to leak at 0.5bar (~7psi) or even lower. Even "new" ones didn't manage much over 1 bar without heavy leaking - i think most of them were produced back when the cars were sold and the membranes are brittle, so IMHO the best solution when going for higher boost is the evo 8/9 RS metal BOV.