Zucchini Bread Problem

The last year or so, I have been having a problem with zucchini bread caving in in the middle. I'm using the same recipe that has worked for me for the last 30 years. I'm baking at 5800 ft altitude in a gas oven. The problem is that the bread will rise properly at the beginning of the bake time, then the center will fall while the edges continue to rise. I am using  2 t of baking soda and 1/4 t of baking powder in the recipe. I have checked the baking soda by adding 2 t of vinegar to 1/4 t of soda. The powder I checked by adding HOT water to 1 t of powder. They both bubbled actively. I checked the resistance of the oven sensor. It was 1060 ohms which is in the proper range for a room temperature check I went through a complete 1 hour baking cycle with no bread, measuring the oven temperature each time the oven cycled on and off, using a thermocouple and meter. The oven maintained the 375 degree temperature setting within 15 degrees, plus and minus. The digital oven temperature indicator said 375 degrees all through the cycle. I would replace the oven sensor just to make sure, but they're $90 to $100 and I'm not spending that kind of money unless I'm sure that's the problem, which it doesn't seem to be. The recipe I'm using for 2 loaves of bread is as follows:

3 eggs

1 c. veg oil

1 1/2 c sugar

2 c. grated zucchini

2 t vanilla

2 c all purpose flour

2 t baking soda

1/4 t baking powder

1 t salt

3 t cinnamon

1 c walnuts

Beat eggs lightly in as large bowl ( I use an electric mixer)

Stir in oil, sugar, zucchini, and vanilla

In small bowl mix flour, baking soda, powder, salt, and cinnamon ( I use a whisk)

Add powder mix gradually  to egg, oil, etc. and stir in ( I use an electric mixer)

Add nuts, stir in

 bake for 1 hour at 375 degrees.

Any help and/or opinions will be appreciated.

The zucchini might be adding too much moisture. One thing I read for high altitude baking is they always seem to require more flour. I'm using white sugar, but then I've always used that so I don't see that as the problem.

Another thing that points to stove problem is that in the middle of this zucchini problem, I baked some banana bread and it came out o.k. It had no baking powder, just 1 t of soda. I also read that too much soda or powder (they seem to do the same thing) will cause the bread to fall in the middle. Since the oven sensor tests I ran came out o.k. I'm not sure what the stove problem could be. We have very few appliance stores with people knowledgeable in gas stoves. The one I talked with. the kid told me my problem was that I was using a gas stove instead of electric. Naturally, I'm skeptical about using THAT place for technical advice. My stove is a GE, only 9 years old. I'm really stumped. Thanks for your help and suggestions.