That is interesting, i haven't seen this before now. My thoughts would be to try a few workarounds/troubleshooting steps to determine where the problem may be.
1) Update graphics drivers. This may sound odd, but I have seen that fix problems.
2) Update print drivers.
3) Create a PDF,DWF and see if you get the same results. Oddly again, I have seen different results going to these two formats.
4) Within Revit's setup for mechanical, you have a 'gapping' setting. Try setting that to zero and test the results.
Keep us posted on what you find.