Volkswagen: Mil code 1128, vw jetta gls, lambda probes

Question
Hello, I own a 99 VW Jetta GLS 2.0L, Occasionally I get a MIL code of 1128.  After doing some research I see thats is a proprietary code that indicates  "Long Term Fuel Trim mult., Bank1 System too lean".  After clearing this code I may drive for weeks before it re-appears.  Any ideas?

Thanks

Answer
we have come across this fault on a few occasions, generaly it turns out to be an air mass meter fault. on rare occassions it can be caused by a faulty lambda sensor. on one occassion, and i mean this is very un-common, it was a faulty engine ecu. so the things to check or try are the; 1)air mass meter  2)lambda probes if both are ok and working properly, the ecu may be at fault. to test the air mass and lambda control you need a diagnostic system which allows live readings / meassured value blocks. failing that, then it is a very expensive trial and error repair.