I had this alot and it drove me crazy for a while.
It could be one thing or a combination of things. It doesnt sound line an EGR problem. Have you read the codes since you put the new EGR on? Have you also read the codes with just the ignition on (postion II)? If not, try it. I code a code that wasnt stored when the engine was running and it related to a faulty Vacuum solenoid (error
P1590 Vacuum Solenoid Valve Circuit Open). See my thread
HERE. This turned out to be my main issue.
The other options are:
Split vacuum pipes. These tend to perrish on the rubber connectors but the engine cover can rub through them. They are best replaced with silicone hoses from ebay for £6. See my thread
HERE.
Boost Sensor. It can get gunked up as you know but you have to be careful cleaning it. It might need changing but you tend to get error code P0234 - Boost Pressure Control Circuit Negative Deviation. This error code could also indicate that the turbo Actuator rod needs a halft turn to adjust when the boost comes in.
read the codes again with the igintion at postion II and with it running as well then post up your findings.
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