They can look fine on the outside and still be no good.
The core becomes brittle and twisitng and wiggling them to remove and
replace them might have separated the core in one or more of them.
You can pull them off and check with an ohm meter but after 20 years I
wouldn't even bother I'd just replace them with a good quality set. Even if
it isn't all your problems the new wires won't hurt.
"Opportunities are spawned from crisis"
I have a 1972 350 4 barrel with HEI, in a 1967 chevy C10 pickup which
isn't driven much. Recently changed Spark Plugs and it runs fine except
under load and then it cuts out but if you let off gas it runs smooth
again. Checked under hood at night and there are no obvious arching
between wires or to ground. Are the wires still the most like culprit.
They haven't been changed in 20 years, Yea I know, but they never
caused any problems. TIA