Nick,
My recommendation would be on the fuel system. Has your fuel filter been replaced recently? Although I don' t think it would impact you at idle, but I would be concerned about running only one fuel pump when under load. If this is a '79 car it should have had both fuel pumps...right? I'm assuming that your fuel pressure was verified and check out ok. Lastly, if your injectors are questionable, I'd replace them. At idle your running less pressure, such that if you have partially clogged injectors your likely to be running too lean on those cylinders that have the worst injectors. You might want to also try posting on
www.rennlist.com .
Good luck,
David
1986 911 Turbo