sierra

I have a 91-1600 pinto sierra-problem is when approaching a rounabout and clear to carry on slip her into 2nd put your foot on the accelarator she just for a couple of seconds will lose power then all of a sudden pick up then run rest of the day,it doesn't do it all the time,it will genrally play up like this when it's cold but once again send her on and it clears,any ideas?changed plugs,leads,cap/rotor arm.