When you say that the results are "invalid", what exactly do you mean? The input/output signaling will be significantly different if you change the input sample rate. I don't know if you mean that the MATLAB sims are invalid or if the hardware was not behaving. In the latter case, you will need to create signaling in and out that will have quite a few blank spaces when changing from 60MSPS to 1MSPS. That is a ratio of 1/2 to 1/120. I find the info below to be pretty valuable when coming up with interface logic. After compiling the design, you can right-click on the FIR filter and select "Help...". The following info will appear. This happens to be for a 4-channel DUC (interpolation = 2). In your case, I believe you would see something like the following. It can be used as a baseline for interface logic.
60MSPS:
<c0><c1> (repeats every 2)
1MSPS:
<c0><c1><--><--><--116 more--> (repeats every 120)
[FROM MY DESIGN / DUC INT=2]:
Input Data Format (Repeats every 48 clock cycles)
<c0><c1><c2><c3><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><-->
Output Data Format (Repeats every 24 clock cycles)
<c0><c1><c2><c3><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><--><-->
If your data looks bad in the simulations, you might need a down/upsample block from simulink to make the FFT, etc look good.
Jeff