I can be cruising along and it will down shift to third gear and stay there for a while for about 7 miles then go to fourth and stay there for about ten then do it again any ideas?

It could be your TCC solenoid switch needs adjustment, here'* a write up. https://www.gmforum.com/electrical-133/tcc-solenoid-switch-284972/ Also have your car scanned for codes. Transmission codes don't always trip a SES light.

Or, it could be temperature based. If your transmission fluid is getting too hot, your PCM will unlock the TCC until the fluid comes down in temperature.

Has it been hot while your driving?

Tired dirty oil and a clogged filter can cause this as well, Yes?

When you say downshift, by how many rpms?
0-500 is going to be the TCC locking and unlocking
1,000ish is going to be a true downshift from 4th to 3rd.

TCC will lock and unlock based on load. Unless you are pulling a 7 mile mountain, the behavior is a bit odd and I'd check the switch that Dan mentions.

Actual downshift is about the same, unless you live in the Alps..it'* odd. Temps can be a factor, but really shouldn't. If they are causing it, then something bad is likely beginning to happen inside your trans.

Could also be a TPS sensor that may be giving a wacky reading to the pcm depending on throttle angle. In this case it would be understandable that the pcm is calculating the load etc and the TPS angle to suggest that the lower gear is where you want to be for the best power/etc.

^ Yea, what Bill nailed it down to.

i can be cruisung at 14-1500 rpms and itll downshift to 25-2600 and i know of its tcc locking and unlocking it does that and in the city it runs about 220 and highway about 195 200 the guy i bought it from told about this issue when i bought the trans and he said the tourque converter was going out so i used the one from my old trans but it still does it

and the filter fluid and gasket were all replaced right after i bought the trans