Just a couple of ideas: 

1) Try setting: 

Infragistics.Win.DrawUtility.UseGDIPlusTextRendering = false;

Do this in the initialization of your app, like in the Form_Load of the main form or in your Main method and see if that makes any difference. 

If that fixes the issue, then this is likely due to an issue with GDI Plus text rendering which is notoriously inaccurate. 

2) Try changing the Style on your UltraToolbarsManager. Note that the Style might be overridden by loading a Style library, so you might have to change the Style in the isl file if you are loading one. If not, then you can just set the Style property in code. See if the problem happens in all styles or just one particular style. 

Hello Veronika,

I’ve contacted the developer team responsible for the control and their response was that, in order to investigate the issue further we would need a sample that replicates the issue consistently.

It would also be great if you could tell us which version of Infragistics controls you are using as well as your environment specifications?

Thank you for your cooperation!

Looking forward to hearing from you.

Regards,
Ivan Kitanov