the motor 4,2 start fine and run fine, but somtimes and now offen. begin the energine go up and down in rpm and i shall give 1/2 to full gas for at the car can acc. normaly.
But when i go under 1/2 gas its begin again. what can be wrong ??
i have one time stop the energine for 30secunds and when i start again the problems where gone

The car have Euro cat 2 install and i have the idea at its this black box ther couse the problems but i dont know, can i cancel the euro cat 2 ? for testing this ?
the cat warning lamp has ligting 2 times for a few secunds but i dont think its the cat ther is gone. becouse its normaly dont light when the problemes come and normaly run my cars fine without any problems.
Sorry my bad english, hope some one can help me to locate the problems.

Regards
Sören