Creating a cosine wave of 140 MHz out of 80 MHz clock

Creating a cosine wave of 140 MHz out of 80 MHz clock

I'm working with Virtex 5 ML507 board. I'm trying to create a cosine wave of 140 MHz out of 80 MHz clock. I'm receiving the data with a clock of 80 MHz and transmitting it with this clock (through the GTX). I want to create a cosine wave so that I can multiply the data and move the signal on the frequency level.
The thing is, I'm not sure I can create this cosine wave with this clock. I tried to create for example 40 MHz cosine wave and got only 2 points on it, for 20 MHz I got 4 points and so on, accordingly to nyquist theorem. I though of maybe of upsampling but I didn't get a chance to work with this block. I'm creating the cosine wave using CORDIC.
Any ideas?