Hi, I got a really strange behavior going on with our '87 300TD gearbox.
It works perfectly fine, but then, recently it would stop sometimes shifting to 4th, then sometimes even to 3rd; just stays stuck in 2nd...
It never happens from the beginning, but rather after 30 or more minutes driving.
We had fluid/filter changed about 1.5 year ago.

It might be something really strange going on with the fluid; I can not see any leaks, but added almost 2 quarts over last couple of months.

One thing that is probably a really stupid question, but I am not really 100% certain how are you supposed to check the fluid level.
As per manual, I check it with warm car running in neutral on flat ground.
The problem is that always one side of the stick will be totally covered with the fluid, looking as there is way too much, while other side will be almost dry, shoving below the min. mark.
I spoke to one mechanic who told me to ignore the high side and fill till I get low side to be within marks; this is how I ended up adding almost 2 quarts....

Now, if I check level when car is cold and not running it shows already more then max. mark; at the same time, when worm and running it is still fully covered on one side of the stick and pretty dry (even below min. mark) on other side.

So, could someone confirm if this is normal and is it possible to check the level on cold, not running car, or is this level totally irrelevant?
Could my problems come from to low or too high fluid level; it seems that adding fluid would help the shifts, but would not confirm this as 100%...

One thing I can confirm, it is not the shift-down switch as this functions as normal; at least when gearbox is working fine.