1993 GMC Jimmy surging at idle
 

Hi all
I've got a 1993 GMC Jimmy with a 4.3 liter engine and automatic transmission. The only problem it has is while the engine is idling, the RPMs go up and down and it stalls sometimes. An hour after driving it it is hard to start too. I've carefully checked for vacuum leaks and found none. I've replaced the o2 sensor, fuel filter, knock sensor and still it idles erraticlly. Our Jimmy has only 180,000 KM on it and has been in the family since brand new and has given us very reliable service till now. The oil is changed regularly and the fuel pump we replaced is the only major work done to it.
Any help or tips would be very appreciated!
Thank you
Georgie