Transmission Anomaly (300E)?

I inherited a 1986 MB 300E w/131k.

The transmission shifts cleanly when cold; and when warm, with good acceleration. However, when warm, with very gradual acceleration, the shift point from 3rd to 4th is less than clean. The engine revs from 2k to 3rpm for about one second before it engages.(It is just fine under hard acceleration). I just had the transmission serviced, although I've had little experience w/ this car before this time. My question is this. How serious in this shifting anomaly. Will I need a new transmission in 4 months or in 4 years? Any one willing to foretell the future?

Thanks,

Steve