RPMS randomly go up to 4,000

When going down the road, no matter the speed, my car will randomly
downshift and go to about 4,000 rpms then kick back into the right gear. If I
let off the gas, it will kick back into the right gear as well. I can go weeks
without this happening, then it will start happening again at random points.
I've noticed that the temperature goes up when my car starts doing this. I
have changed the mass air flow sensor, transmission fluid (no metal
shavings in it), transmission filter, all the air filters, and put injector cleaner in
the car. It is throwing some codes, but my code reader says that they are
manufacturer codes. They are as follows: p0030, c342D, C3064,
C2174,C2520. It didn't seem to start doing this until I replaced the flex pipe
on the muffler, however I have no idea if that has any correlation. Someone
told it could be a shift selenoid. Up until it starts to downshift, it doesn't shift
through the gears hard at all. It doesn't really even shift hard when it starts to
mess up. I am at a loss. If anyone could help,it would be appreciated.