Hello,
Has anybody had this happen or any clue to what it might be?

I recently replaced the coils, wires, and plugs. The engine was running beautifully after I did. It has nearly 260,000 miles, no rough idle, and it had good power when accelerating. I say had because I began to notice a slight power loss, not much, about a month after the repairs.
Then this happened. I am cruising along at 65 and suddenly it feels like my engine slips gears, from overdrive to third. If I accelerate it does not shift. There are no strange noises and the slipping is very smooth when it happens. If I put my car in neutral while I am driving, turn it off and then start it again; after engaging drive again it shifts into overdrive correctly. It also seems to do it more on hot days or after driving it 20 or more miles. When I said I noticed it slightly lost power while accelerating, it is only in the gears above third.
I am hoping this is not leading up to a major transmission problem.
I also have an O2 sensor that has gone out. Do not know if this has had a factor in it. The engine still runs great besides this and I would like to get another year (more if possible) out of it before I decide to lay it to rest.
Any ideas or thoughts on this would be greatly appreciated.

When was the last time the fluid and filter was replaced?

Oil has been changed every 4-5k miles. Oil and air filter, every other oil change. Fuel filter, never. Did a radiator flush about three years ago when I changed the water pump. Transmission fluid is still runny and light-red. Trans has not been flushed or filter changed in 4+ years.