sounds like you have an air leak.
Look at the clear palstic fuel lines while its running with a bright light and look for bubbles. Even lil ones.
The trick is to identify in which line they first appear.
The fuel path is as follows: from tank into fuel thermo in the cly head, then to top of pre-filter, out bottom of pre-filter to shutoff valve (SOV), out bottom of SOV to end of lift pump, out lift pump to main filter, out main filter to SOV and finally into IP. Detail
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=210025&M=606%2E962&GA=722%2E608&CT=M&cat=19T&SID=07&SGR=120&SGN=04
Find which line has air and then look at the seal(S) prior to that point. Usually is prior to fuel exiting the lift pump as it is suction to that point. Check the Delivery valve seals on top of the IP too.
Each line is sealed with an oring. Sizes are here
OM606.962 o-ring size or buy "viton" orings from fryerpower.com.
The white horseshoe clips on the ends of the fuel lines are spreaders; push them tightly closed for removal, loosen them to seal.