Power off issue

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Maybe this has come up before, but this is now one long thread and I was not successful in the search mode, so I hope you all will bear with me.

I have a 3 year old 880 which has given good service (once replaced for the infamous charging issue) and does all its chores, replacing 9 remotes. I am having no battery or charging issues, 3 bars all the time. What has just begun to happen is that in the Full shut down mode (using the large chrome button) from TV viewing, the remote will shut off the TV, but not the Preamp/Processor, even aiming right at it from one foot away! The issue corrects with the trouble button each time, but this is a pain. Changing the timing setting (lowering) as recommended from the website does not help. I will say that this problem does not occur each time, but about 60% of the time which is really puzzling. (I have also not changed any power-related settings in the 3 years!)
The processor is a McIntosh MX119 and is also not having any issues.

Anyone got any ideas?

I can offer a few suggestions... starting with some very obvious ones that I'll apologize for now..
1) SORRY... Did you make sure the remote and the space where the IR receiver is on the Processor are both nice and clean?
2) If you point your Harmony remote at the Proc with everything else already OFF does it work more reliably? That may be a clue that you are getting IR interference from your TV or other devices
3) Do you still have the original remote from the Proc? Does that work 100% of the time? If so, that would help rule out an IR receiver issue.
4) Have you ever changed the 880 rechargeable battery? Even though it is being read as "full" the battery may be worn out to the point where "full" is not what it used to be. You can check this by seeing if the remote can hold a charge as long as it always did.

Thanks, Jim,
I did do your number 1, but will also try the others, all good suggestions. Strange that "off" in the only command the processor cannot handle and that all was good for years until recently. Will do those tests as you suggested. Much obliged!

This issue is now apparenlty solved. Adjusting the command delays seems to have made the difference. Jim, you pointed me in the right general direction, so much appreciate.