PO305 error code on 1999 Jeep Cherokee

Jeep has started to idle rough. OBDII code reader says PO305 (#5 cylinder misfire). Changed plug and wire, no help. Problem only at idle.

Asked by for the 1999 Jeep Cherokee
check the injector and do a compression test. lots of problems with exhaust valve seats going bad.

Compression check done. O.K. How do you "check" the injector? What about the EGR ?