90 S15, new eng, idles rough, low power

okay....here is my great mystery of the day.
90 GMC S15, all sensors and throttle body, egr, tps, injectors, fuel pump and filter all relativly new and in good working order but due to
high km's and loud knocking it was time to replace the engine.
ordered a rebuilt engine, transferred over all parts, everything went well and it was installed.
started right up and ran with no problems and idled smooth.
Then....went for a drive, and as soon as things warmed up and hot normally hot I noticed I didn'd have much power, I could barely get the truck up to 100km/hr without having to floor it....which was odd.
Then....I came to a stop and it was idling rough...bad miss it sounded like.
checked all the usual suspects from plugs to wires etc, timing.
I notice that when it is cold it runs fine....can smoke the tires if I want going out of my driveway but as soon as I warms up it is hardly driveable and rough to almost the point of stalling.
Mechanic after charging me $3000 cdn for the rebuilt and install now says it might be a small leak on the intake manifold/gasket.
I am not convinced but have run out of ideas. We are currently installing a used intake from a scrapyard just in case but I am not sure this will solve the problem.
I am watching and making sure the use a torque wrench and follow the bolt tightening sequence.
if anyone has any ideas please email me or post, I am able to recieve and reply to email on a RIM/mobile device.
and if anyone can recommend a better shop than the one I am at please do....I think honestly I have more experience that the oil and lube techs that pass themselves off as mechanics here.
thanks in advance Mark Toronto Ontario Canada
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

If it runs fine cold but runs like shit hot, I would be looking towards an EFI/sensor problems as oppposed to a mechanical issue like the intake manifold leaking. Leave it together and start truobleshooting the EFI system.
First thing is first, have you checked the ECM for DTC's yet?
Doc
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Wild guess-Catalytic Converter partially plugged?
--
RJ in WV

Saw on a bumper sticker.....
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.