170k miles. Check engine light goes on, but it doesn’t tell mechanic what is wrong and has him puzzled. The problem temporarily went away a few months ago, but it’s back and I need some troubleshooting before I try to get it worked on. Brother-in-law thinks it’s the throttle position sensor, but thought I’d see what others say. Don’t want to get taken for a ride.

Each time I start the car all is fine. Within 5-10 miles when I am at any cruising speed I start losing speed/power and my tachometer starts wobbling (even at idle now sometimes). If I push on gas pedal harder nothing happens, but if I tap the gas, it clears up most of the time. Sometimes I have to tap it a couple of times to get it to run smooth again. And sometimes the condition comes back within a minute or so. Once, when going up a gradual hill, no matter how much I tapped I could not get the acceleration to kick in and chugged up at 30 MPH, but that only happened the one time. Other hills have been okay. No rhyme or reason around consistency, except this last time it seemed to start again after I filled up with regular (as usual) at my normal station, Costco. I did put in fuel injector a few months back and the symptoms disappeared shortly after, but I was told that isn't what fixed it at that time. I wonder... although it seems something more serious is going on and I don’t want to lose all my power at the wrong time.

I get my car serviced every six months and believe all regular service needs are up to date. Ideas?