I have an '85 380SE which has been driven seldomly for the last few years. The last time I had driven it, it seemed normal. However, now the car will only shift up to a higher gear once its at around 5000RPMs. So, when I'm driving on the highway getting up to speed 80-100Km/h I had to push it all the way up to almost the red zone (which starts at 5500rpms) before it will switch up to the next gear.

Any suggestions on what I should take a look at? I tried to check some of the vacuum lines (which seemed normal with a cursory inspection) but it would help if someone could point to the particular one/s to look at and any other possible things to check.

Thanks in advance for any suggestions.