This is a link from the MBCA club website where several members were trying to solve a boost related problem in a W124 OM602.962 car. Assuming your boost control system is the similiar take a look at post #4 in the following link.

http://mbca.cartama.net/showthread.php?p=57707#post57707

According to Jtripp over on the MBCA site the EDS computer looks for a reduction in engine airflow during EGR operations. The reduction in airflow is expected since EGR operation introduces a small amount of gas from the exhaust system. If the specified reduction in airflow is not "seen" a EGR fault is assumed.

Have you checked the vacuum source to the EGR transducer and the vacuum supplied to the EGR valve? As I recall EGR functions are also temperature and RPM related so faulty sensors in either of these areas could cause problems.