I have some additional information in case anyone is still following this thread. I believed I can safely say that the problem is not with the ADC's or the Demultiplexers. First, I connected the same triangular input to all four channels, and enabled only channel one (a bad channel). Photo one shows the output A7 of U850, the ADC for channel one. In photo one the bottom trace is the input signal, the top trace is A7 output of U850. As can be seen, bit A7 changes exactly at the zero crossing of the +/- 2 Volt triangular wave input. When each channel is enabled individually, the corresponding A7 bit or each ADC responds in a similar manner. This, I believe is what one would expect since this bit responds to the polarity of the input signal. Now observe the displayed waveform on the TDS540, photo 2. The basic triangular waveform is present, but broken up by the seemingly random spikes. Nothing in the output of the ADC for channel 1 would indicate that these spikes are being caused by anything forward from the ADC. It is interesting that the output B7 of the Channel one ADC is identical to the bit A7 output. This is true for all channels when enabled individually. Not sure why the two outputs for each channel, I suspect it has something to do with the demultiplexing.
Speaking of demultiplexing, I also believe that the demultiplexers are working just fine, as most of the signal (all of the signal for channel 2) appears as it should for all channels (except for the spikes. Since all channels are turned off except for channel one, I would expect that if the demultiplexers were messing up, any demultiplexer screw up would cause the displayed signal to have any spikes back to the zero level, which is clearly not the case.
This leaves the memory, or memory control, but this would mean that there is bad memory in three of the four channels, keep in mind that channel 2 works just fine. Indeed, this seems to eliminate the control circuits. It is just difficult to believe that three of the four channels would develop bad memory together.
All comments are appreciated. I am sure I am a victim of 'fuzzy thinking' here, but cannot determine where. Any ideas on how to prove it is a memory problem? Or not a memory problem?