Problem in generation from 3D pdf

I am trying to generate from a 3D pdf:
MEGRDLikCorrelation corr_RD(“corr_RD”,“corr_RD”,Ep,Eg,costheg);
datagen = corr_RD.generate(RooArgSet(Ep,Eg,costheg),numevents);

I find strange steps in the correlations of the generated variables e.g.

Does anybody knows why this happens?
The output of generation says:

[#1] INFO:NumericIntegration – RooRealIntegral::init(corr_RD_Int[Eg,Ep,costheg]) using numeric integrator RooAdaptiveIntegratorND to calculate Int(Ep,Eg,costheg)
[#0] WARNING:NumericIntegration – RooAdaptiveIntegratorND::dtor(corr_RD) WARNING: Number of suppressed warningings about integral evaluations where target precision was not reached is 1

thanks in advance