Just a thought -- I assume you torqued down the head nuts radially starting from centre and working outwards. Have you re-torqued now the engine's warmed up/cooled down a few times? Otherwise the usual stuff...
Leedsman.

 

As you had head gasket problems with #5 & 6 you might have a plugged up catalitic converter (the ones that are in the manifolds plug up then forces the ehaust back up the manifold which creates heat and slugish engine responce). I had similar problems and resurfaced head twice before finding the problem was caused by the cats (i just punched the guts out of them and put them back and the emissions still checked out ok I guess the ones in the pipes worked). The best way to check is put the car on a lift remove the manifolds from the exhaust pipes,start it up and see if there is equal flow out of the manifolds.