[morzh]

OK, when back to it:

when the radio works fine, establish the setup with a sig gen, pass the signal, make sure the AVC reacts. Variate the strength, try to make it react, if it does not.
Measure the amplitudes (if you have scope, use scope, if not, use tracer; or use a meter, AC Volts) of the signal along the way:

1. Cap of 6A8
2. Plate of the same
3. Cap of 6K7
4. Plate of the same
5. Plate (any) 6H6.
6. Connection between R5 and R7 (Detected voltage)
7. AVC voltage (top pin C2 or left pin of the R5)
8. All the 3 taps of the antenna secondary (which you measured before together with C2 and the cap of 6A8)

Now, when the radio craps out, with the same exact signal in, re-measure/re-trace the above.

If possible, do both measuring and tracing in each of the two situations. Tracing requires subjective assessment: write down what you hear.