Q: Coolant does not return to radiator after cool down

Original problem: Temp would rise sitting at stop light but would return to normal when moving. I figured the coolant was at its limit and had it "flushed" and filled at sears. New Problem: coolant will not return to radiator after cool down. Car does not overheat unless i do not drain reservoir back into radiator manually. I drive 1 1/2 hours to work everyday and the reservoir will be almost full everyday. I had a suspicion sears did not "flush" the system so i did it myself and back flushed a golfball size gelatin goober out of the system and it did pretty good for 3 months or so with very little filling of reservoir and now the problem is back again. Brought it to a mechanic and he said he sees nothing wrong except my tranny was starting in 2nd and getting to 4k rpm (which is what it does when u put the CVT into manual shifting) so i told him he was a moron and should learn how to drive the car before trying to work on them and am now looking to find answers. Thanks

My car has 155000 miles.
My car has an automatic transmission.