Kia Sportage 4X4 problem, very strange

Hello, I have some very strange problems on my car. Car is KIA Sportage 2.0 TD 83HP year 2000 4wd.
So I will basically start to explain you the problem hope we can get a fix :

Sometimes even if it is warm or cold engine rpm indicator starts going crazy jumping up and down, but engine does not actually increases rpm in reality it just becomes sluggish. Car feels like it looses power and its shaking back and forward but when I put the gas pedal to the floor it tends to get better and the rpm indicator doesn't go crazy anymore, and engine runs more smoothly but I can still fell the loss of power. It usually happens when engine is warm and after 2000 rpm. I had the following errors on the OBD1 :
Intake Air Temperature Sensor.
Engine Coolant Temperature Sensor.
Atmospheric Pressure Sensor.
Pressure Regulator Control Solenoid Valve
Idle Speed Control Solenoid Valve.
Distributor no Signal. Or Injector No. 3 open or short. (Not sure which one)
Power Stage Group 2 Malfunction (inside ECM). Idle Speed Control Valve Failure or Damaged Power Stage.

What should I do? I am not a mechanic but I know friends that are but they are not experts on kia, they work with other cars from the same year, different brands. So what should I try to change first? Do I have to change all those things?

Country: Norway
Make: Kia
Model: Sportage
Year: 2000
Engine: 2.0 TD 83hp 4wd

Hello, first I want to thank you for helping me, you really know your job :) Unfortunately this car is European model and on the european models diesel from this year they have only odb1 connector. I am 100% sure. I have read the errors using the paperclip metod, and it worked, and I found how to read the codes thats where I got them from. So should I start buying all the sensors? Or first clean the trottle then check the battery and clean iac hole too and check fuel pressure, and after that check errors one more time to see what I will still get. The car is used for newspaper delivery, that means that I stop at every mailbox at every house leaving the car idle while I put the newspaper in the mailbox, and then 1 gear, 2 gear and go 20 m and stop again, and I drive it like this for 5 hours every day. When I don't work with it I try to rev it up a little and press the gas and when it goes after 4000 rpm I see some gray smoke coming out of the exaust. I rev it up like this 2 times a day 3 rd gear and until 5000, because I think that it cleans itself somehow. Is this ok? How should I maintain this car giving the condition that its used 5 h a day at newspaper delivery. Should I clean egr more oftenly, and change oil and filters more often that normal? And one more thing when I put some diesel cleaner aditive in the tank the car has power again and doesn't have the bad simptoms so often. What can this be related to?

Sorry if I make grammar mistakes but I am from Norway and english is not our native language. Thank you again for everything. Waiting to hear from you..