Can a very high mileage ( 150,000 miles) timing belt stretch enough to slip one tooth so the the engine still runs but not full power? A friend said her car was not running well so took it to mechanic who took off a cover and said her timing belt was bad and had slipped. Soon after the belt broke.
Yes, it is possible. The same could also happen if the belt tensioner is old. The symptoms will be as you described or worse.
I had a problem like this on my car, turns out there was a recall for my specific model of Honda Accord because there was a design flaw in the timing chain and/or tensioner
If I had registered the VIN with Honda when I bought it, the nearest Honda dealership would have fixed it for free but I didn't until years later so had to shell out for it myself. I did this before the belt broke however
Hope that helps