I have implemented double buffering but I still get some flickers - any ideas ?

Your video card settings should have some sort of verticals syncing options… play around with those…

my TNT2 has some buffer swapping setting too…

I’ve tried your suggestion - but to no avail

Any other ideas?

How have you implemented the double buffering? With double buffering turned on and no VSync, there can be a horizontal… erm, “folding” line (in lack of a better word) on the screen where the memory addresses are swapped while the cathod ray runs down the screen (results in e.g. having one frame in the top half and the next frame in the bottom half of the screen). Actual flickering you actually shouldn’t get.
Sure you’re still drawing everything in the back buffer?