I'm very sorry for the long delay in this response.
This was our third visit to this mechanic. On the first visit, they saw that the signal from the crankshaft position sensor was poor and erratic, so I replaced this sensor. I noticed when I removed the old sensor that a 3" section of the sensor cable (where it passed over the valve cover) was carefully wrapped in black electrical tape. The only times anyone else had done any work on this engine were about eight years ago when a dealer took care of one or two recall issues, and about four or five years ago when the oil pump, water pump, and timing belt were replaced. After replacing this sensor, the problem remained, and the ECM again showed camshaft position sensor problem codes. We took the car back to the mechanic, and they stated that both sensors' signals 'looked' clean on their oscilloscope. They suggested replacing the camshaft position sensor as an experimental repair and charged me nothing for this visit though I know they spent at least two hours looking into this. I replaced the camshaft position sensor. Afterwards the problem was still present. This lead to the third trip to the shop. After reviewing the history, and taking a test drive they looked for a restriction in the exhaust. This car has two catalytic converters, one immediately downstream of the exhaust manifold, and one under the car upstream of the muffler. As the downstream converter's bolts were easier to release, they looked at this one first. They found that though there was no apparent external damage, the ceramic catalyst support been breaking apart and collecting in the outlet port of this converter,gradually restricting the exhaust more and more so you were also on the right track.