Mackie Onyx 1640i + Cubase 5 delay

Hi guys, a beginner here.
So, I’m currently using Cubase 5 with a Mackie Onyx 1640i in a 16GB Windows 7. I’m using a driver buffer size of 128 samples and a sample rate of 44100. Here is what happens:

I’ve properly set up all things - I can hear, I can play and record, everything works fine. My current problem is that I’ve created a new project consisting of 2 Kontakt tracks (drums and bass), 3 guitar tracks, 2 voice tracks and 1 effect track (total of 8 tracks). I recorded my guitars 2 days ago perfectly - I could hear them instantly with my monitor button checked (the yellow one). Then, I came back today to test my microphone to record my voice. Now, I get a little delay when I sing - but that’s not the first time that this happens.

In other words, the first time I open the project, everything is fine (I get no delay with the monitor button and I can record it perfectly). But, if I get to close the project and open it again, when I check the monitor to hear my voice, record additional guitars or anything else, I have this boring delay of a few ms that makes it impossible to record.

Any ideas?
Thanks in advance.