Cabin heating issue - 1993 W124 300CE

[optimizer]

Hi!

I have an issue with my cabin heating.

Here are the symptoms:

- Start up the car on a cold day, drive until the engine warms up a bit
- Turn on the cabin air and set the temp to the mid-70's (should heat up the car)
- Blows cold outside air without heating it up

Here's how I fix it (strange.. but this 'fix' works consistently):
- Sudden acceleration. I'll be sitting at a light, and on green I'll jab the throttle a bit, and suddenly, the air coming out of the vents is warmed up! No kidding, I've tried this on more than a dozen occassions and it works every time. If I don't do this, the air doesn't typically start blowing warm by itself.
- I've tried to see if sudden braking would also be a fix, but it doesn't seem so.

Any thoughts on the problem and a permanent fix??

Thanks!!