I can almost guarantee that the ECTS sensor has gone wonky. There is a 1 wire sender for your temp gauge on the dash, but the 2 wire sensor is for the Ecm. The sensor in my 2002 sl1 crapped out and i experienced the almost the same issuse yiure having. Did okay when it was first started, but after about 15-20 minutes of driving, it ran like crap, and F/E suffered greatly.
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