Eurospec 84 alfetta sedan, cough, sputter, backfire troubles

["dnaj" <dnaj@domain.elided>]

My 1984 European spec Alfa Romeo sedan is sick. The drive train is 
basicly the same as the common spider from the era, that being 
4cyl, DOHC, Bosch FI, 5 speed, save with the European two piece 
exhaust manifold/exhaust and no smog. I replaced the plugs, cap, 
rotor, about 1000 miles ago and had no problems. Now my problem 
is this...

When letting off the gas at most any speed, the car backfires.
When letting off the gas after hasty acceleration, the car 
backfires quite loudly. When not accelerating, but only trying
to maintain speed, the car/engine shudders and shakes terribly.
Acceleration is fine, no problem driving straight up a hill or
accelerating on a hiwghway but lifting off the gas so slightly 
will cause shuddering. Lifting off the gas completely causes hard 
backfires.

At idle the engine runs fine at all RPM.

If first assumed the very bad resonator was not providing enough 
backpressure. So I replaced the Y pipe and resonator with the 
correct European spec exhaust. (Try finding that for a sedan!) 
Now the entire exhaust has been replaced or repaired with OEM 
parts.

But my driving problem still exists. I quess the backfires blew 
the resonator and not the other way around.

I'm at a loss, please help! Thanks
dave in eastern pa. (pix of the car at www.users.fast.net/~dnaj)

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