'93 Jimmy 4x4,4.3,W : EST

Well I've read over alot of posts about this code, but I thought I try to put out another one:
I posted earlier about my truck making a whineing sound. It ended up
being a bad Intake manifold gasket. I had my shop do it, but a week later, I started to notice a lack of power. Then out of no where the truck begain to shutter, idle real bad. Also when your stopped, and accelerate the engine hesitates, chugs away, then kicks in. This happens between 0-15 mph, 30-40 mph, and 50-65 mph. Since it happened in the middle of the night and week, I slept in the car at the mechnic and gave him the truck, exlpaining that it was spitting a Code 42: EST, and the symptoms. He was confused about the code (It didn't help we were experiencing unsual 110 degree temps in Northern CA) He suggested that the plugs be replace. Since it was due for a tuneup, I let him go ahead. The truck ran fine, for 2 days. No code, no symtoms. Then this morning it was a hard start, and after the truck heated up, it begain to hestiate and the CES light came on; Code 42 again.
When I open the hood and open the throttle I hear a burst of air from under the vortec cover. Could a disconnected vaccum hose be a culprit. Where should I go next?
To make matters worse my other truck a '83 Jimmy's water pump just failed, 2 Jimmy's in 24 hours... Forcing me to rent a car til I figure out what I'm doing.
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Google code 42 plenty of info out there on this.

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