how did the old plugs look like? did you replace the dist cap, rotor and wires - if not would be a good idea to inspect them.
since you're saying it's only at a certain throttle position makes me think is the TPS but I can't be sure. 
any codes (check engine light)?
I knew I was going to get called out on all that.  lol
Old plugs looked ok.  Nice tan color...
Yes I have a code, No I haven't had it checked yet..  I need to do that just not much time.
No i didn't change the wires but I did inspect them and they seemed to be ok.  I could always change them out for good measure.
No I didn't change the cap and rotor.  Nor did I inspect it.  Ran out of day light..  I plan too as soon as I get a full day off work....  
But as for the TPS I thought that sensor effected your idle aswell..