It's hard to tell whether the SEO plugin had any thing to do with your problem, as you don't describe the time sequence of what's happened, and you don't indicate what you had the plugin do. I'm wondering, eg...
- When did the SEO plugin come into it?
- Did the site always use the SEO plugin to rewrite the title, or is that something new?
- Also, did you use the plug-in to change all of the titles on the site at the same time?
Can you give us a sense (using example terms like widgets, gizmos, kwd1, kwd2, etc) of what pattern your titles had (ie, the titles you liked)... and how long they were in place before Google rewrote them.
Chances are that neither WordPress nor the plugin has anything to do with why you've gotten rewritten, but it sounds like the choices you made are hurting you.
In my experience, titles that get rewritten are generally too long and stuffed with keywords, or too short and not descriptive of the content or function of the page. There's nothing wrong with keywords in the title... that's perfectly natural... but long lists of keyword variations that exceed Google's default display length are a technique that's not helpful and will cause Google to change the title.
If Google wants to display the company name in your titles, that might indicate that Google thinks your company name might help click-throughs. I'd look upon that as a positive sign.
This thread discusses some of the things you're observing....
Google is now rewriting all my page titles http://www.webmasterworld.com/google/4480232.htm
[webmasterworld.com]
The topic has been discussed a lot here. I'd use Google to search webmasterworld.com for threads on the topic, using searches like...
[google rewriting page titles site:webmasterworld.com]
