Why is my exhaust manifold glowing red?

A week and half ago my van started
losing power like I was taking my
foot off the gas on the expressway. I
had it towed to a muffler shop where
they put a new catalytic converter
and first muffler. Two days later The
van was running rough when I first
started it. The exhaust was
sputtering. Last night I detached the
second and final muffler. The van ran
fine on a 15 minute test drive.
Tonight on a 15 minute drive with the
muffler still detached the van ran
rough and the exhaust manifold was
glowing red again. No check engine
light, engine temp reads fine.
Changing spark plugs tomorrow but
that seems like a long-shot. Any
ideas?