1994 Toyota Landcruiser wont turn over, sometimes

My truck is very very reliable since I use it in colorado at times. It turns over 95% of the time. However, sometimes I turn the Key and nothing happens, will not turn over. I walk away and come back ten minutes later and it starts. I checked the battery, alternator, and all I have left to check is the starter and the ignition switch. Starter is not making any noise. Some people told me it could be the old aftermarket alarm causing the problem.
I hope you can be some help
Thank You
David