1996 3.4 V6 surging at 4,000 RPM

Hello I just had a rebuilt long block installed. We replaced all the sensors as part of that job. It was running fine for about 200 miles, then last week on start up it
showed a check engine light and was running rough. The following morning it was the same. Later that afternoon it ran fine. Ran fine all week.
I took it in to the shop that did the engine and the code was random misfire.
Didn't do it again until tonight. It was running fine..went into a restaurant...came out and it did the same thing. If I run it up in neutral it runs rough but runs until 4,000 rpm. at 4k it hiits a wall. It drops to about 3600 and then runs back up to 4000 under steady throttle. . if I give it more throttle it surges harder and the rpm swings are greater, but it will not run over 4,000.
I was able to drive it home..but it idles and runs rough.
This is in a 96 Silhouette minivan.
Any thoughts other than I should never have gone through the brain damage of putting a motor in this POS?
Thanks for any insight
Peter
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The PCM has a rev limiter of 4000 RPM in park or neutral, so what you observed there is normal and nothing to do with the problem.
I'd suspect an ignition problem to start - are the plug wires new?
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Robert Hancock Saskatoon, SK, Canada
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Yes - new plug wires and new Platinum Plugs.
Ok - I didn't know about the rev limiter and that would explain it. So what I have is a rough run and idle. When I said new sensors, I was referring to the block sensors and coolant. I did not replace any of the intake system sensors. Did clean the injectors.
The dilemma continues...
Peter
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