I'm going to disagree with Dan on the issue being the clutch. The clutch pedal being slightly depressed give it the opportunity to slip and damp out the bucking.

I would have him look at the carburetor float level. I had a new car that surged at highway speed years ago. I took it in to have the dealer fix it and the service writer swore that it was because "the spark plugs had to seat" and had to "get a glaze on them." I kid you not! Knowing enough about engines I challenged him and took my case to the service manager. It turned out to be a simple float level adjustment. Never had an issue after they corrected that.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.