I have a 1985 MB 190E 2.3

It has 215,000 miles on it. It has an erratic idle when it warms up...(1,000-1500+ sometimes) and it fluxuates. Can it be my idle speed control valve? There is also this screw under the air cleaner in the block which i can adjust my idle with but it never stays normal ( I dont think its supposed to be there ) When I put it into gear at high idle, the tach will drop to about 500 or less and struggles and gets back up to about 1000...and in reverse at high idle, it actually peels out! (embarrassing) This is my main problem with the car.
I also have a problem with my electromagnetic fan...
I think the clutch or thermostat may be bad...the fan is constanly on...it never shuts down to its standby slow rotation speed...help on this would be greatly appriciated! Thanks!