Absolute newbie here, apologies in advance. I’m having a weird issue with my 1994 WRX. I’ve had it for about 18 months and it’s been good as gold minus a slight coolant leak. I recently had it MOT’d and some bodywork done on it and my mechanic noticed it was only boosting to 0.5 bar. He suggested I get a different wastegate actuator which I did off eBay. I installed it and for the first 10 minutes it was boosting to 0.9 bar absolutely lovely, then started massively overboosting (1.5-2.0 bar) so that it started dropping out and bucking. Baffled I swapped back to the original actuator which then did the same thing!
I drove to a mechanic nearby and it was overboosting every time I tried to floor it, but after turning the engine off for 5 minutes it started holding at 0.7-0.9 again. Drove it for about an hour holding boost at 0.9 then RANDOMLY back to overboosting.

Not sure at all what’s going on when it’s overboosting even though the wastegate actuator hasn’t been adjusted. Any ideas?!