Have 230E 89 and have a problem

Going at 3rd gear at low speed such as at 55Km/h or below, as soon as i take off my foot from the gas pedal, it shifts 4th. Why is this? But if i keep the foot on half of gas pedal it shifts at 80Km/h to 4th and that is normal. Would adjusting vacuum modulator help in this case? In fact this problem is for all gears not only 3rd.

this car should be driven at high RPM or Low? Which one is better? I usually drive it at 3000 rev.

Would changing ordinary ATF with good fluid(synthetic) and filter help in smooth shift.