Same thing just happened to me. The car was slowing down for no reason.
First it slowed up .05 for no reason. Ran the valves, looked over springs, drained oil and cut open filter. Nothing.
Next race slowed .10 1st round for no reason. Put it in the box to look it over at home. Plugs were rich but O2 didn't show it, then O2 would go to 15.98 just running it at idle and off idle up to 2k. Shut down ignition and restart would be fine for a while.
Next race go day early to test. First and second runs O2 wacky. Install new O2 sensor and start to get good info, pick the car up .2 but it's still .1 slow. First qualifier picks up .02, second slows .05, third another .05 I'm lost. My buddy comes over and tells me to go back to basics and check timing. Do it just to please him. It was 16* advanced. Match up what timing should be as shown on the laptop to actual.
What do I dial first round? What map do I run? How long has the O2 and timing been messed up?
Go back to test session where the car ran deadly consistent and load fastest map. Use that run vs. weather, and fastest run of the weekend vs. weather to come up with a dial. It's a .14 window. Throw a dial on the window in the middle. Kill a few and run over but flat out would have been dead on the dial. Lucky guess. The car should be a little faster but it's a little fat for what it likes.
Next time out will verify timing and tune it up, should run normal numbers.
This year has been a learning experience. Earlier in the season the car would speed up or slow down .05. Found that to be a rear suspension issue. Fixed that then the O2 and timing gremlins show up.
They say they come in 3s, so things should begin to get back to normal. Maybe!
