92 Astro surging/cutting out

I've got a 92 Astro van with a"W" in the vin. I put a new engine in 2 years ago, and it has about 15k on it. The van has run like a top since the new engine was installed. Yesterday I was on the interstate doing about 60mph, and the van surged twice, acting as it would if the electrical power was cutting out(just and explanation). It then was alright for the rest of the ride home(20miles). Today I start back to work(30 mile trip), and after going about 15 miles it does it again, but this time it kept doing it, and cut off. I pulled it to the side of the road, and it did start back up, but I had to give it alot of gas. The van runs flawless under 40mph or so. It idles flawless also. It only seems to happen at interstate speeds. My mechanic could not duplicate it around town, but he did feel it once on the interstate. He seemed to think it may be a transmission problem, so I had a trans guy ride with me also. The van operated fine with him at the wheel, and he said it could only be one of 2 things. A torch converter/stall converter, or some type of engine problem. Thank you for any help you might lend. My wife's medical issues take almost all out extra money, so I'm trying to get a handle on this myself.
John

I would generally discount the possibility of a torque converter or other transmission problem for now. Whether the converter is sticking in a locked condition or failing to lock it should not cause engine stalling at highway speeds. Obviously, I have not driven it so if there are other indications we are not aware of them, so based upon the info given I'd look elsewhere.

More common suspects might be fuel delivery (pump, filter, regulator, poppet injectors), restricting exhaust, or ignition overheating/intermittent failure. If there are no stored error codes, you might want to look at fuel and ignition first, since they are not very closely monitored in the older ECMs.

Another remote possibility is the EGR valve. It should open under moderate driving conditions at highway speeds, but if for some reason the ECM is losing control over the EGR and it is opening fully and failing to close upon load, that may cause a similar symptom.

Thanks my friend. I'm just getting back to you because of being so busy. The mechanic has had my van for 3 days, and has taken it out for several drives, and is unable to duplicate the problem. The last time it happened to me, it smelled like something was burning. This mechanic has always been good at fixing my vehicles, but he's trying to do all his other work, and because he can't pinpoint the problem; he hasn't made progress on mine. It is my work van, and I can't even transport the equipment I need to and from jobs. The van operates flawlessly until you get out on the poen roas above 50 or so, for a sustained period of time, although my guy couldn't get it to act up. I just don't don't know where to turn from here. Thanks for any more suggestions.
John

After reading your above reply again, I remembered that my mechanic did have to put a new ignition module in the van about a year ago. When it went out though, the van wouldn't start or run at all.