Many Thanks to everybody for their supports,

Yes Alain, it may be impedance problem or Offset voltage drift in Op-Amps but it is not.
As I said before, I tried many techniques and schemes, I used OP27 (very low offset) with symmetrical power supply and I always precisely check that kind of voltage drifts. Then I used MAX6241 (4096mV ref with a single supply). Sometimes I used a series resistor(1-4,7k) in analog inputs and/or/not capacitors (100-470 nF) between these pins and GND.
May I ask you, did you ever precisely measure the voltages with a 1 mV (or lower) resolution voltmeter? If everything is okay may I see the summarized ADC section of your
code (if it is in PBP)? Because I'll be insane soon.

Thanks Darrell Taylor, I know, this code was one of the test code combinations.
I read the manual's A/D section and desperately tried many clock setting parameters and
also ADCIN command. These definition lines should be forgotten to be deleted.
Staistical sort method is the last way, I used this technique only on the Maxim's high speed
(100-200 kHz) 12-16 bits A/D converters. There was no need to use in 18F252.

There must be a structural difference between 18F252 and 2520 to affect their using technique and resulting difficulties.

BrianT, you are right, analog ground distribution is very important to prevent the voltage drops. I use protoboard for now but 18F252 was direct working in every condition. I don't understand this.

Namely, I compare everything with 18F252 comfort and then getting angry.

Can anybody write me a simple summarized code including definitions, settings and ADCIN usage. (I don't need high speed conversions. I want slow but stable and true results )
(for 2520,4520, 2550 or 4550, all they have same A/D structure).
And talk about differences between 18F252 or F452 and other new versions of these chips?

Then, once again, I will try it on a PCB.

Thank you

Selim