Help with bad shudder please!

'98 4x4 5.7L Tahoe. I bought the truck from my ambulance company last week. It has a bit over 200k miles on it, but they put a brand new longblock engine in it for me before I got it. It's got many new parts, inlcuding distributor, cap, wires, plugs, crank sensor, ect sensor. New fluids, hoses, belt, radiator, fuel filter. Timing is within 1 degree of TDC.
Now for the problem... When accelerating, there is a bit of hesitation, almost like bad gas. When just feathering the gas pedal to maintain speed, the truck shudders and bucks very badly. It feels what I would think it would be like to throw the truck in Park going 55 for just half a second. If I start accelerating again, it won't do it anymore, but as soon as I ease off the gas again, it'll start. Truck does idle absolutely perfectly.
I should mention there was half a tank of gas in the truck that was probably about 6 months old. I filled the other half of the tank from the pmp a couple of days ago hoping that would help, but no luck.
Any ideas????
Thanks, Adam.

Well, I wasn't able to get any help on my problem here (although I doubt anyone would have expected this problem!).
Took it to a shop, and $500 later it's fixed.
Apparently, when the mechanic put in the new motor, he didn''t install the harmonic balancer bolt tightly. It backed out, allowing the harmonic balancer to back out. This allowed the veluctor (which gives the crank pos. sensor its signal) to come a bit loose.
The only reason the shop even found it is they noticed the crank sensor had a little mark on it upon removal. They put their pinky into the sensor hole and wriggled the veluctor.
Just a follow up in case anyone else has this issue...Worth a check, especially if this problem occurs in a new motor!

Glad to hear you got it solved. So how's it running with the new long block, now that you can really drive it?