Very odd behaviour of preamp tube in vintage Gibson amp ("dancing voltage")

[g1]

Then I tried the OT resistance method as per Helmholtz's advice, but I did not succeed because I could get no ohm reading between the center tap (probe on the first filter cap or on the rectifier lug, alternatively) and either tube plate. My Fluke multimeter gave a flashing 0.L (=no reading). Why is that?

Power off and filter caps discharged?

[Helmholtz]

Then I tried the OT resistance method as per Helmholtz's advice, but I did not succeed because I could get no ohm reading between the center tap (probe on the first filter cap or on the rectifier lug, alternatively) and either tube plate. My Fluke multimeter gave a flashing 0.L (=no reading). Why is that?

Reason is the large OT inductance in series with the DCR. Some meters are having trouble with that.
Short the speaker output (secondary) to short the inductance while measuring primary DCR.

[slidincharlie (Carlo P)]

The DMM (a Fluke 77 III) was on Ohm setting (not VDC Tom Phillips am I correct on this?), the amp was powered off with filter caps discharged and unplugged from the mains.
Helmholtz: should I short the wires going to the speaker while reading the resistance from the OT center tap to either tube plate??
BTW, the DMM should be okay (I hope!): other resistance and voltage readings are fine.

[Helmholtz]

Helmholtz: should I short the wires going to the speaker while reading the resistance from the OT center tap to either tube plate??

Yes.
Nothing wrong with the meter.

[slidincharlie (Carlo P)]

Reason is the large OT inductance in series with the DCR. Some meters are having trouble with that.
Short the speaker output (secondary) to short the inductance while measuring primary DCR.

Done. My readings with the OT resistance method match the "mean" reading from the cathode voltgae drop method: 33 mA from each tube.
If I did not make any error the tubes seem to be matched.

[slidincharlie (Carlo P)]

End of the story: I increased further the filter caps value (33 uF instead of 10 uF in the 2nd and 3rd filtering stage). The motorboating went definitely down. Not disappeared bu much weaker, and occurring only at high Intensity settings.
Thank you guys for all the advice.