this will probably be due to the fact that the output frequency of the mac is not exactly 75hz or 60hz.

are you genlocking to anything?
turn off genlocking if you have it on.

this will/might be part of the scan convertor design - and its attempt to lock to its own version of 60hz or 75hz.

you will have to try different scan convertors.

you see exactly the same thing with a DL1 -when you run the input at 75Hz.
sure it will take 75Hz in. but internally the device works at 60Hz so you get exactly these problems.

this isnt normally an electrical problem its a design problem in scan convertors.