Greetings, I'm hoping to find some help here (aren't we all?)

We have a Paloma tankless water heater sold under the Rheem brand via home Depot.
The model number is ECO-180DVP and it is an inside LP model.

It has been running very well for just over a year. A few days ago it started acting up.
First let me give a few details...
We live in central Arizona in a small town that has its own spring fed water system. Our elevation is 4,650 feet above sea level. The water system is an old one that originally built by the mining companies. The mines closed in the 1950's and the town has been running the water system since.
I mention this because we often have problems with the water supply when the watershed receives a lot of rain/snow in a short period or when there is a break in the water supply requiring repair by the town crew. Several days ago we received a good rain and the mountain springs began to fill up the storage tanks with a great deal of "turbidity" (a term the town government uses when the water coming into the storage tanks is agitated by the increased flow). The result is we get a lot of tiny bubbles in the water for a few days and we often get small particulates (dirt) in the water supply.

Okay... now here is what is happening with the water heater.
If we turn on a hot water faucet to very low (enough to fire up the heater), the heater will stay lit and provide hot water.
If we turn the faucet up to medium or high volume the heater remote shows that the heater has turned off (no flame) then restarts for about 20-30 seconds then shuts off with a "12" error code.
We have checked the water supply filter on the unit. In fact, yesterday the unit would not stay on at all so we checked the filter and found some small grains of what looked like sand. When we cleaned the filter the unit again would function but only under low water flow.

Prior to this recent change in the water supply it had been working fine.

Any ideas or suggestions would be greatly appreciated.

All the best,

Noel