Sounds like we've had the same car...
If the clock is losing time and the seat-belt warning buzzer is coming on randomly, the problem is likely to be the voltage regulator (and maybe a bad battery also).
New regulator can be bought for $40 at NAPA. Bosche is higher... check parts shop for regulator VS re-manufactured alternator it may be cheaper in the long run to get new alternator.
Have you replaced the fuse(s) for the back up lights and the low fuel warning light? The problem could be simple... what have you definately eliminated as a possibility?
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles