Losing torque at 2k RPM

I drive a 2005 Mazda 3i 2.0L with a 5-speed manual gear box. About two weeks ago I had the car serviced for a misfiring cylinder (#3 if it matters). They diagonsed the problem with one of those On-board diagnostic dealies because there was no noise, or change in performance but the check engine light was on.

They replaced the coils and the check engine light went off, and the car was running great–not that anything was wrong before that.

Anyway, I’ve been driving it about 5 miles a week for the last two weeks without a problem. Today I drove it about 40 miles to and from a nearby town. As I got close to home I noticed that I was losing torque when the engine was revving between 1750 RPM and 2500 RPM. Above and below that range the car was fine, but in that range it was sluggish and unresponsive. A few minutes later the check engine light came on.

The mechanic that worked on the car ran another test (who, after starting the car, discovered that the check engine light went off). The cylinder was firing in time, the coils were fine, the spark plug was fine, the compression was fine. They couldn’t find the problem.

They are now checking for carbon build up and inside the fuel injector but everyone’s stumped.

Anyone have any idea what might be wrong with the car? Is there such thing as a fluke? Thank you huge in advance for any help.

There would most likely be a history code stored for the mysterious check engine incident. If it is not a misfire some searching will be requires. My WAG is a intermittent fuel pressure drop.