After trying to troubleshoot, reading this forum, tand having a technician out, the problem can't be solved and hoping for help here.

Equipment: carrier 58yav 110btu installed 2001. Installed in house with clay chimney flue shared with water heater. Unit installed with carrier chimney adapter designed for use in clay chimney. Chimney adapter contains standard bvss switch correctly installed in series with dss flame rollout switch and pressure switch. Installed McLean, Virginia. Clean filter, unobstructed air ingress/egress, clean single speed blower seems working fine.
History- We bought house 2005- unit had no problems until about 2009
In 2009, we had one DSS flame rollout reset in the season. Igniter also replaced this year
2010, 2011 - 2 or 3 roll outs switch reset each year
2012- last few weeks, persistent flame rollout reset after about 10 minutes of running with heat/burners fired.
2012- pressure switch replaced with factory part
2012- this week, replaced dss flame rollout switch, correct factory part

Prior to having technician out I had chimney inspected (also did myself), cleaned inside of unit carefully, measured rpm in inducer motor speed to be spec at 3400,inspected inducer motor internal fan through flue exit at top of unit and is very clean, and burners look clean. Very little to no rust.

Tech came out and measured temperature at dss rollout switch. He determined it was getting too hot. He didn't understand the chimney adapter operation entirely, but he noted that when he temporarily restricted the air entering the chimney adapter, the temperature across the rollout switch decreased. He called his supervisor and advised that "you need a liner for the chimney" but they also agreed that the chimney was in specification for the chimney adapter. I also asked why we would need to do this given that the unit worked ok for years. The tech then inspected the heat exchanger from up above by looking down from the ac coils installed above unit. He felt the heat exchanger looked very good. Neither he nor I could see obvious flame rollout in operation. As last effort, tech adjusted gas pressure valve to try to reduce gas pressure. I think he said that the air coming from the unit was a little too hot as well. This did not solve the problem. Technician left telling me to get a liner and he would remove chimney adapter. Final observation is that the chimney flue vent pipes that run for about 6 feet and 2 elbow turns should be double walled and now it is only single walled. Unclear if that is correlated to issue and if so, why it would appear as a problem now 11 years post installation.

I have run out of options and greatly appreciate any help you can provide