Intermittent Ignition on my 1987 LR 90

I'm a novice owner trying to learn. My 2.5 TD starts most of the time, but randomly the starter won't engage. The starter was rebuilt six months ago and I have cleaned the connections to it and checked the battery is good. When it won't start I can try the key 35 times and get a "click" and then it will arbitrarily start normally on the 36th try. Any ideas? Has anyone wired a switch as a go-around? Thanks

Check all your engine/body earth straps and clean them too, could try using a jump lead from battery neg to engine, if all that fails it's probably the solenoid.