Chevy Venture Hesitates on Hills, Any Ideas?

Hi,
I have a 97 Chevy Venture, 3.4 L engine.
I had the known intake manifold gasket issue at 75K miles, and had to replace the engine.
Ever since it was replaced, the van hesitates in 4th gear
going up hills, just before it downshifts.
This is very frustrating, cause it only happens after about 45 minutes of drive time, so most garages think it runs great.
I suspected the transmission, but a shop said it was fine. Next, I thought electrical, so I replaced the wires. This helped a little, but it is still driving me nuts! Finally, I unplugged the EGR valve to see if this was the culprit, but it still hesitates.
Any suggestions would be greatly appreciated. (and no, I cannot sell it or donate it, though I would love to, but it aint in the budget...)
Thanks in advance, Kevin
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Sounds like secondary ignition misfire or valve body. Hard to tell which from the description, and they feel nearly the same.
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