Debug specific problems in Excel / Visual Basic program

I have an Excel worksheet that utilizes a GUI and inputs data into an Access database. It was created using Visual Basic by a very talented Freelancer who has gone on to bigger and better things, and for the most part works excellent. There are a couple of glitches or bugs that occur intermittently that I would like to have corrected. The GUI inputs system time and date when the "start" button is pressed, and inputs system date and time again when the "stop" button is pressed. The interval between the two times is then calculated and input into the proper field automatically. One example of a bug is that sometimes the system time displayed in the GUI upon pressing "start" freezes, and so when "stop" is later pressed, it shows zero time has elapsed. It always inputs the correct start time and I can manually edit the fields later, but it is frustrating because it should be so unnecessary. If I can get just that bug fixed it would be a tremendous help. I can provide a copy of the software if needed, or we can use remote access in real time and I can show you what I mean. I will also provide a list of software also running at the same time to screen for possible interference. The main problem is, while I can easily show examples of past occurrances, I can't reproduce the bug with any consistency at all.