I believe there are many reasons for odd transmission behaviour... however, mine started to do exactly what your car is doing, and it meant a £1000 rebuild.

I had water getting into the transmission fluid through the radiator.

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190E's:
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
http://www.maylane.demon.co.uk/190esmall.jpghttp://www.maylane.demon.co.uk/190esmall2.jpg