you have to put the colder plugs in it and i think they need to be changed more frequently than the stock grade ones do.
if you ran with stock grade and it was good at one time maybe you just need them replaced because they're worn out.
it also could be a vacuum leak. its 98 so you probably should redo all of them anyway. autozone or advance or oreilly has it by the foot. its like a dollar a foot. and get the plastic t fittings.
or they have fancy silicone online with brass t fittings on zzperformance