Have you watched the AGC voltage as you tune across the band? I suspect the
AGC is reducing the IF gain when you tune through the mid-band, due to a
stray oscillation. Could be caused by a bad capacitor on the AGC line. A
scope should show it.

The AGC seems to be working, since the reception is good on a weak
station near the lower end of the band. I can't imagine the AGC would
work at some points in the dial and not at others.

I did manage to get good reception in the middle band by adding another
antenna loopstick and capacitor. I used a few turns from the external
loopstick to couple the signal into the radio main loopstick with
another few turns.

Two loopsticks works better than one, but they won't both fit into the
same box.

-Bill