Thank you all. My strong desire is to resolve the Insteon issue and maintain the investment. The suggestion of moving away from Insteon is only relevant if Smarthome's solution of rebuying newer devices is the solution (which I doubt).
For Paul's questions:
Q-Did you have a power outage or spikes around the time the keypad went bad?
A-Not that I'm aware, but it is possible.
Q-Are the manually linked devices all near their keypads? This would explain why they worked.
A- the distance varies. But they are the same devices that are having comm issues. However, in response to LeeG, they manual linking I performed at the direction of Smarthome Tech Support was the simplest of links.
Q-Is the PLM relatively far from the keypads in question?
A-Within a 30 Ft radius. The installation is a converted commercial building, so it is a 25'x50' box on 2 floors. I lost contact with all 2476D devices which are installed on both floors.
Q-Is the PLM near anything noisy like low voltage lights, a TV, a motor of some kind?
A-It is where it has always been, in a closet with an Elk panel. Regardless, I have a router on the second floor, so I am going to move it there just to try that angle
Q-Is there any chance that something happened to your PLM, did you replace it recently?
A-I suppose this is possible. The PLM with the 99i system did have to be replaced once during the 99i days. This PLM came with the 994I kit so it is relatively new. Is there a way to test a PLM or should I just replace it to rule that out?
I failed to say earlier that I have:
1. moved the keypad to another location (two actually) without any change in behavior (StusViews suggestion), and
2. removed the 2476D devices that were having issues and re-added them; also without any change in behavior.
The system is on release 4.2.30.
Thanks again for your suggestions.