Lowish resistance between transformer windings

[atw58]

I'm trying to find the cause of severe ringing on a switching power supply, the transformer has four sets of windings. Resistance is OL between all sets but one (primary to one of the secondary) which reads 1.2M. I've read 1M per 1000V should be the min insulation resistance of wire for motors etc. The max transformer voltage is 400V pp. The rest of the components look good; cap esrs are fine, diodes ok, resistors ok.

Should I disassemble the transformer and fix the low resistance.

[shakalnokturn]

Was that measured in circuit or off PCB?