Many computer cables are pretty straightforward to diagnose with any el-cheapo DMM that has a continuity check feature. (Continuity checks help determine if there are breaks in wires in a given cable.) Just put the multi-meter in that mode (which may be "diode check" mode on some DMMs) and put one problem on a pin at one end of the cable and the other problem at a pin at the connector on the other end. It's even easier to do this if your DMM beeps instead of just giving a readout of zero, that way you don't have to keep your eyes on the meter when you check.

The reason I say this is because you don't always have to purchase new equipment to solve these problems. If you just like to have spare parts, that's fine. But many people by old Mac stuff on EBAY, which can often be rather high priced (for decent, clean equipment). So if you are a classic Mac lover, even if you aren't an electrical engineer or technician, a DMM would be a good tool to have around the house, just as a hammer or screw driver would be.