Transmission Slip Out Of Gear Problems of BMW 323

I bought a 1999 BMW 323i in January of 2000. It was a certified BMW. I haven't had much trouble with it. I have done typical maintenance, brakes, oil changes, new plugs, wires, etc. Last year I noticed while driving at speeds more than 65 miles per hour that the rpms would move from 2500 to 3000, and a light showed up on the dash. I got to work and check my manual and it is telling me the light is a service transmission light. If you see this light, pull over, put it in park, pull the emergency brake, wait and restart the engine to reset trans. I took it to my repair tech and he plugged my car into his computer and it states that the transmission is slipping out of 5th gear. Estimate for new trans is $4,000. For a rebuilt trans is $2,500. 00. I don't have that kind of money for a 10 year old car. I have noticed in the winter months (cold keeps trans fluid thicker??) that it doesn't slip as often, and in the summer months, if I keep my speed under 65 mph, it doesn't slip as much. But it is a hassle, and I am just waiting for it to get worse. It slips out of 5th gear while on the expressways making it extremely dangerous to pull over and reset the car so that it will go into 5th gear. I don't especially like to drive it when the rpms are so high, it seems to be putting a strain on the engine, and my gas mileage has got to be decreasing tremendously, when it slips out of 5th gear.

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