Lately I've noticed a bit of roughness between 4000-5500 rpm, somewhat worse under load, and set aside some time this weekend to inspect the plugs, coils and connector boots, check for oil leaks in the plug wells and decide whether a slightly early tune-up is in order. The plugs were fair for their age (~44,000 mi), not great, so in a few weeks I'll go ahead with the tune-up and replace the valve cover gasket as well--the #4 boot was oil-fouled and replacing it cured some of the roughness. My worry is the #2 and #5 plugs would not come out. The plugs broke free easily enough then became increasingly difficult to turn. I stopped trying around 30 N-m or so (the tightening torque is 25 N-m). I tried working them back and forth a few times and gained a little ground, but still met considerable resistance. With no spares on hand I couldn't risk snapping an insulator or, worse, damaging the heads, so I re-torqued them to be dealt with another day. I'm hopeful this is just some anti-seize compound galled up in the threads--some other plugs also felt easy/hard/easy as they came out, but not to this degree. I was working on a cold engine so I'm also hopeful that if I tackle these two plugs first with the engine hot, they'll spin right out. Any other tips on how to deal with balky plugs without giving myself a heart attack (I likes my cylinder heads, I does, and plans to not hurt 'em)? Also, if the tune-up doesn't cure it completely, what else should I poke at to smooth out the upper RPM range? Engine is an M54 2.5L with about 207,000 miles, in otherwise perfect running order, no obvious cracked hoses or vacuum lines, but I haven't thoroughly checked all that in a while (will do so during the tune-up).