You may need to loose weight.. (Ok that is not exactly a joke)
Go to the local hardware or auto parts or other store that does this (Yes Wal-Mart can make a Workhorse igintion Key) and hand the key buy your ignition key, Say "Spare key please" and use this spare.. BY ITSELF, in the ignition, Do not hang other keys from it.
One of my favorite jokes about driving and living in a class A is to say "Only one problem with these class A's" and when the other party says "What's that?" I hold up my key ring and say "They all fit".
Which, by the way, they do.
It is a big ring.
The weight of all those keys can cause strange wear patterns on the ignition cylinder and in some cases the switch itself. Causing the switch to not fully return to "RUN" and thus causing the symptoms you cite.
Home is where I park it!