Excessive FD spring tension


Zog The Undeniable

So I got a braze-on added to my frame for the front mech (sorry Sheldon)
after the band-on one gouged the tube. The shop sold me a NOS 1057
mech, which is mid 1990s vintage but in theory should be fine with my
105 triple chainset and Ultegra "8 speed" friction bar-end shifter.

Except it isn't. The spring tension on this thing is so huge that it
makes the cage flex under the pull of the cable. I aligned the mech
with the cable unattached but found the rear of the cage flexes
downwards about 3-4mm, and scrapes the outer ring, when the cable is
fitted. This was solved by moving the mech higher up the slot but, much
worse, when riding out of the saddle in the middle ring the cage creeps
inwards, presumably because BB flex is giving the cable a series of
little tugs (it stays put when riding on the flat).

The shifter has never been lubricated - ISTR that's always a no-no for
a fraction shifter - and is certainly not loose. I've ordered a "9
speed" 105 FD which will hopefully solve the problem by being designed
for the lighter action of STI shifters - but is there a known
incompatibility here?