transmission trouble

Hello all. I'm looking for opinions on the following problem that my 2002 Focus Wagon (automatic transmission) is having: The transmission slips badly between all gears when driving in stop and go
traffic for awhile and on hot days. When the transmission slips, I can push the "over drive off" switch and the slippage quits entirely. I've had the transmission flushed and serviced but the problem remains. The tranny shop wants to put a new transmission in for $2300 but I feel there might be a less expensive solution since this condition only exists when the overdrive is engaged and it's hot or I'm in stop and go traffic.
The Focus has only 21,000 miles on it.
I'm thinking I might have a faulty control module or solenoid. Any ideas or suggestions? thanks, Tom
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