After a week of debugging and analysis, I am reaching out to the development community. http://www.horsepowerfreaks.com, our company's online store, is experiencing a major drop in sales since our recent update. Plenty of testing has been done to try to recreate the errors we see in our logs, but we have been relatively unsuccessful in determining the root of the problem.

History:
This site was started back in 2002 I believe by our CEO/Owner. I assumed part of the responsibilities 4 months ago, and immediately began revising the front-end to catch up with the times. It was more of a patch though. The process was quite rushed because, well, once the boss realized I had some abilities, he wanted a new site "Yesterday". Regardless, my 10 years experience in development has led me to this position, and I am learning new languages as I go along. So debugging the ASP Classic that our system utilizes is difficult at times. I've always been a designer, not a developer. As of today, my ASP is pretty sharp, jQuery is decent, etc. I am new to the Windows Server environment (which we use). Always been an Apache guy, so finding the root of the evil here has been even harder.

Enough about me, just wanted to explain some of the difficulties of the system so everyone has an idea what is going on behind the output HTML. I know the code is still "Development" quality, but it's what I'm working with.

The Issues:
Symptoms range from 404's (which can be due to many things like hidden parts, old external links, etc.), but also randomly "chopped" URLs (which we can't find why/where this is occuring). We have recently revised our URL parsing file. We think we might have fixed this particular issue, but I doubt it. Site has been tested (limited) on legacy browsers. Most of our users are IE7-9. Mainly IE9 and iOs. On our part detail's (point-of-sale) page (/partdetails/) we do use custom dropdowns that I created. They use DIVs and JavaScript/jQuery to operate. I know if people have javascript disabled, then poof, no sales. But I doubt that's the case. (Gonna check Analytics actually to see the numbers here)

Help Needed:
What I'm asking for is, for anyone who has a few minutes, to browse our site, give any feedback on functionality/errors/issues that you encounter. Add some items to the cart, enter a vehicle when your on different pages, and see if you can find any broken processes. I cannot begin to tell you how many different ways we have tested this system. It's like there is a "Phantom" issue that only occurs outside our local connection. Our site is hosted in house, so it is possible the issues only occur on the outside . Not sure if certain Internet Security Suites could cause issues with our cookies/scripts?

I really need some help here guys, I am at a total loss. Even though this is a major brand's site, I'm still only one guy. My boss can help when he's free, but for the most part, it's on me. We're spending thousands on SEO soon and really need to find out why sales are down before then. Thank you so much in advance for any feedback on the site! I hope a more experienced developer out there has some insight to share with us...