1991 Jaguar XJ6 sometimes crank sometimes nothing

When I try to start my car sometime it will turn over, but most times it will not, not even a crank. I put a knew battery. I checked the ignition on relay (fine) When I jiggle the key it will make the instrument on the dash board lights up and flicker like crazy but no crank. I have an antitheft alarm, I believe it is aftermarket, I believe it may have something to do with my problem. Also I purchased a new ignition switch which I was going to replace this weekend. Please help with any advise that may get me back on the road of recovery.