boots are cracked or split. That let's moisture into the plug and causes this issue. If the injectors test out OK -- you'll need to do a good job cleaning up the plug connector contacts per the posts below, then use dielectric grease to lube everything up for final assembly.
I've repaired cracked/split dust boots by degreasing them well with some isopropyl alcohol and then applying automotive black RTV. Best to apply it after the plug has been reconnected. This repair seals the boot and lasts a pretty long time. But the best thing is to cut out the old connectors and splice in some new ones -- just run a search, many posts talking about this...