Hi, Akurth,
You might just want to follow the answers to this similar thread.
viewtopic.php?f=3&t=13965Admittedly it's an older thread that started out last year, but the last 2 posters (m1hanna & mcmasterc), both from this month, are theorizing that it may have something to do with high heat days (one thinking it may be a fail-safe protection when indicated battery strengh bars are low) & just note that all the posts are in July & August (typically very high heat months).
Sure, your car is high mileage & it wouldn't be unusual to think that something inside the shifter is worn out, but a couple of the cars in that other thread are either young (one year old for the original poster that started the thread) or with fairly low mileage compared to yours. Still firepa63 may have a valid point in regards to your high mileage (therefore high use/high wear & tear) shifter. Let us know of the final outcome/solution.