Engine surging for first few miles
 

1st generation 1988 2.8L Blazer automatic. I recently started the restoration project on this and have purchased a manual for repair. That and my wits are all I have. So I pretty much have a manual and a very limited tool box. However, I am motivated so give me some help and I promise I will follow through. Here is my problem. Engine cranks fine. Runs fine sitting in the driveway. Once I start down the road and reach highway speeds for approx. 2-3 minutes the engine starts to surge. The RPM gauge shows a jump of about 500 rpm's. After around 5-8 miles the surging stops and the truck runs perfect. I want to say that it never seems that it is going to die. Only that it is surging. Also, I think it worth noting that when the A/C is turned on the engine loses performance. It seems to drag the power down considerably and cycles on and off to a 5-10 second beat. That seems awful fast to me. Full tune up has been performed by a mechanic before purchasing. One that I trust to be telling me the truth. I personally have replaced the fuel filter. I have been over every vacummn line and they are all connected and seem to be in good shape. I keep reading about the EGR valve and have done my homework on how to take it out and clean it. I am thinking that I should just replace it but before doing so would like to ask advice. So, what do you think about my problem?

Have you assessed the health of the catalytic converter? A clogged cat can sometimes be ok when the engine is cool, but once things warms up, it can really restrict flow.