97 Jag. XJ6, 4.0, 120K miles

Recently, my car won't start easily when I turn off the engine for less that 30 minutes; I assumed it needed new spark plugs. After I put in new plugs, there was a sudden loss of power. Now my mechanic says it is the head gasket. He says 2 out of the 6 cylinders are not working. Prior to this spark plug change, the engine never over heated, and it was driving perfectly. When I drove it home from the shop, upon opening the hood, I noticed the tail pipe (the section close to the engine) was red hot. The engine never heated past the half-mark, nor was there any smoke.

Any other thoughts as to what could be the likely problems?

Many thanks for any help.