I've ran into situations like this before, works one day, doesn't work the next. It has almost always been a hardware problem. Poor connections mostly. But one time it was the actual cables I used to connect the 8451 to the device under test. The cable impedance affected the quality of the signals. Cross talk can cause a problem. That is why you need a scope to look at the signals at the device end.
In my case, it worked one day, and the next it didn't, it was because the cables were moved and crosstalk started occuring. Also, the clock signal was skewed by the cable and it no longer looked like a clean square wave at the device end. You may need to change cables or use very short wires (less than one foot in length). Keep the wires apart. If using ribbon cable, surround you clock line by grounds. Surround your data lines by grounds. It almost certainly has to be a hardware problem.
One time I had a problem because I was using level shifters to transform 5v to 3.3v. The device did not like the output impedance of the level shifter circuit. I had to play with the resistors on the output side. A scope will go a long way to help you troubleshoot.
Sorry to hear about your colleagues giving you a hard time.
