Might be you need to change your pre-filter and main filter, when is the last time the filters were changed? It is possible that the primer pump has begun leaking, they will pull air into the line when that happens, the promer pump can be had through FastLane and PartsShop for a song.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.