Hi Guys,
I've got a similar problem with my 91 Jetta diesel - runs OK when tank is full, but when it gets below 3/4 I have trouble starting it and at times runs rough. I think the pump is loosing it's prime (air getting in line). Any thoughts on what you've found??

Check all the hoses from the tank to the filter to the pump. If one leaks it actually can suck in air as there is no electric pump & they are not under pressure. Does it still have the "clear" plastic hose, I've had them leak before.