I'm working on returning a 1970 220D to its former glory. In the past year I've replaced its fuel lines, rear brakes (everything), fuel filters, etc., and rebuilt the vacuum pump. I thought we had smooth sailing ahead when I completed the vacuum pump rebuild last month, but I was wrong.

I've been struggling for the last few days with a new problem. Wed / Thurs of last week the car was hard to start. Took 10-30 minutes for it to finally start. Cranked and cranked and finally caught. I figured I might need to change the fuel filters again, as they were both pretty nasty when I replaced them about 2 months ago. So I went ahead and changed them out. The small, primary mainline filter had a fair amount of crud in it (black specs / particles). The large, secondary filter that is enclosed in the fuel filter housing wasn't noticeably dirty, but I changed it anyway.
Drove it around Thursday night to make sure everything was good to go. No issues to report. Went out Friday morning to drive to work and the car wouldn't start. It would turn over, catch / start for a second, and then die. Checked to make sure I had bled all the air out of the fuel filter housing, etc. Kept trying for 30+ mins before it finally caught and stayed running. Drove a block, stopped at a stop sign and then hit the gas to pull out. It died.

By this time I had run the battery down, so I walked to the service station across the street and borrowed their booster box. I actually pulled the big secondary fuel filter out, looked it over, refilled the canister (put Diesel Kleen in there to mix with the diesel that was there), rebled it, and tried starting it again. After 10 mins or so she finally started. I let it sit there and idle (rough then even, rough then even, etc.) for 10 mins. Returned the booster box and drove away. Got it up to almost 70mph on my way to work to give it an "Italian tune-up" in case carbon was blocking an injector or something. Drove fine. Came out after work (~9 hours later) and she started on the second try. No issues driving home.

Saturday morning I went out to try to start her again, with no success. Just wouldn't start. Worked on it for about 3 hours yesterday (Sun.). Checked all the fuses, etc., to make sure the glow plugs were heating up. Couldn't find the 8 amp strip fuse that I'm told goes to the glow plugs. Did find other fuses where the strip fuse was "supposed" to be, so I replaced them. Pulled and inspected the secondary fuel filter again. Removed and cleaned w/ diesel the oil bath air filter. Cleaned the air filter housing out (it was quite dirty), drained the old oil, refilled it, replaced the air filter. Couldn't get it to start. Tried and tried and eventually ran the battery down. Hooked a charger up to the battery and went inside.
Came out to give it one last try at about 10pm. After about 10 minutes it finally started. I let it idle for 10mins or so and then drove it all around for about 30 mins. Parked it and went to bed.

This morning I came out to start it to drive to work. You guessed it - she wouldn't start. Tried for 30 mins., with the assistance of the battery charger boost function. Turned over, even started for a second but immediately died.

What the heck is going on? It has 3/4 of a tank of gas. I filled it up on either Tues or Wed. Do I have a tank of bad diesel (if there is such a thing)? Are the new filters preventing enough diesel to get to the engine when I'm trying to start it? But if its a filter / bad diesel problem, why does it run fine once it starts? If there's a problem with the glowplugs, why does it start some times and not others?
I'm a bit at wits end. Just when I think I'm making progress and getting things fixed, I hit a new wall. What am I missing here?