Strange engine

[a-soares]

I there, I have a 96 Polo with a AEX Engine fitted. The proble i'm facing it's so strange i'm looking for some help trying to find out what is wrong.
It fires up OK and apears to work fine for about 5-7 minutes or so of normal driving. After reaching the normal working temperature it starts to misfire and turn the ride into a rapid sequence of pulls and pushs. After some time (about 30 minutes or so) it return to normal again and stays that way for as long has the ride may take.
What the ..... could be the problem? any ideas from anyone?
The problem desapeard in the summer but returned with the first winter rains.
What i've already done with no success:
I've already checked the connections and none seems to be loose.
I've checked if the ECU is not loose and the cable is well attached (but i didn't took it out to see if there's any pin corrosion)
I've took the distributor apart and cleaned all the connections
I've replaced the spark plugs
I've check all the wires, but i'm not shure what sensors are connect were (a schematic would be apreciated...anyone?)
I've already pulled some hair off my head
:confused::confused::confused:

[twig]

... is that last comment trying to cover up premature baldness? ;)

Check the cold start sensor or any temperature sensors. I don't know the exact names and locations of any of them, but my wife had a similar problem on her golf a couple years ago.

And where are you located?

[a-soares]

no baldness problem, at least not yet :D
I'm in Portugal.
I'll check the sensors, but i only know about the water temp sensor wich is located near the exit/entry water hose on top of the engine. I'll try to find some repair manual online. Any help?
Thanks