Google Ranked Dropping on old High Ranking site

I have a site, very old, that uses html for desktop and Dudamobile for mobile devices. The site is 15 years old. The keyword is not very competitive and is very geographically located just in my city.

The site has ranked #1-#2 position in Google all these years. A few months ago, it started fluxauating all over from #1-#4 position. Last week it dropped to #5, then the other day to #7, now this morning to #10. Now competitor pages, FB pages, CL ads, spam pages and Yelp ads show up higher.

I own 2 other non-related web sites for the same keyword that rank solid #2 and #3.

I'm trying to figure out what is up with this? Such and old site should have some type of authority. Granted by today's standards the site is very outdated and does not have a great deal of content. Would upgraded to a Wordpress site and adding much more content help at all? I don't know what else to do at this point. 15 years is a long time to all of a sudden lose all your organic ranking. My Google maps listing still pops up #1. Could this be some type of Google penalty?