1973 280SEL 4.5 running rough

My 1973 SEL 4.5 is running rough, like when a plug or two has fouled.
It has run fine for the last 6 years, and as soon as I put a for sale
sign in the rear window, she decided she's unhappy. Installed a new
water pump and a new distributor, plugs and points about 2 months ago,
took some time but got the distributor in the right position and she
was running as usual. About a month ago she started kicking down about
30 seconds into warm up, which is good, but the whole time I have owned
this car (6 years) it has never kicked down until fully warm, 5-7
minutes of running. That was my first clue something was up. Idle was
still strong.

2 weeks ago, she fouled 1 of the plugs that was installed 2 months ago.
Took them out and cleaned them up all but 1 was dirty, put them back
in. She ran fine for a week. Plugs fouled again, 2 different ones from
the 1st this time. One was clean, looked like unfired gas, the others
were black and sooty and 2 were actually "fried" if that makes any
sense -- the contacts were uncleanable with a ?carbon? hard black
residue.

Oh, and 2 small pieces of rubber gas line were replaced in the last
couple of weeks as well. Just thought I should mention it.
All were dirty, so I took them out and replaced them with plugs she has
always liked, NGKs. I figured it must be because she has never run for
long on the Bosch plugs. Put in the fresh plugs, started her up and she
turns over but barely runs, as if the plugs are fouled. Give it gas and
it's just as bad, sounds like there is some low *almost* wanting to
backfire in the exhaust.

It's not drivable like this -- suggestions? I need to sell her because
I need the money for a cross country move. Anyone here want a 280SEL
that has run like a dream up until the last couple of weeks?
:-)

I just got back from the parts store with some different gas line, as I
think they gave me line that isn't for fuel injected systems, and a new
fuel filter, just in case.
Alex
1973 280SEL 4.5