injectors not firing

what could cause my injectors to pulse good sometimes and other times they wont pulse at all ive checked them with a noid light

Which year? There can be a world of difference on how the injectors work between early XJ's and late XJ's.

It's an 89 renix

The Renix fires the injectors separately on the plus side and share a common ground. Best guess would be an iffy CPS, do Cruisers CPS test and also do a CPS ohm test. The CPS can be temperature sensitive and work well when cold but screw up when hot.

The injectors ground goes through numerous splices/connectors and eventually ends up at the dipstick holder. An ohm test while shaking the harness might tell you something.

There are other reasons, but these are a good pace to start and the more likely causes.

If you are testing them while cranking, the CPS won't trigger the injectors unless the motor is turning over well (some say around 300 RPM). If the motor turns over too slowly it can cause issues with the CPS/ECU and functions like the injectors firing gets iffy.

I ohm tested the dipstick grounds with them unhooked the other day and one read 00.5 ohms but the other shows 38.5 ohms