improving the sinc(x) interpolation

I need to improve the interpolation algorithm

I'm working on the sinc(x)/x algorithm... and I'm having some trouble understanding some effect I can see on the scope.

in the picture below I'm displaying on CH1 the interpolated signal, while on CH2 I'm displaying the FPGA buffer

the first  picture is is a sinewave 1 MHz  as you can see the sinewave is a bit wobbly...  the problem is that if I use a linear interpolation to display CH1... then it looks not too bad ( see old post)

The wobbly effect is more evident when I use a triangle wave... as you can see... I do have some problem...

now I'm running out of idea... any suggestion is really welcome!