Two possibilities come to mind. Many water heaters of several different brands mfg. during the mid-90s were equipped with a dip tube which deteriorates and crumbles into small bits. When this happens, the incoming water bypasses most of the heating process and after just a little hot water, you get cold water, although there is actually hot water in the heater, but cold is being drawing off. Have you been seeing white bits and crumbs flowing out of the hot water spigots, or clogging the areators? If so, this is a good indication that the dip tube is the culprit. It only costs a couple of dollars to purchase a replacement, but the replacement labor can be extensive, depending on the particular installation.
The other possibility is one or the other of your elements is burned out and you are only getting the heater partially heated. With the power turned off to the water heater, check both elements for continuity with a multi-tester. This will show which, if either is burned out.
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