My Upsells are Fluctuating Wildly...

Yeah, that's not normal at all.

I would start with running some technical analysis on your upsell page(s) to make sure they are functioning properly.

STEP 1:
If you have any video or VSLs running for your upsells, make sure the player is working properly on all major browsers. For example, I was having a conversion problem the other day on one of my membership sites, and when I tested out loading the pages on different browsers, I found out that my embedded VSL wasn't loading properly in Chrome. It ended up being a JS conflict between a WP Plugin I had and EVS (Easy Video Suite). I disabled the plugin in WP and all is good now.

STEP 2:
Run some browser shot tests over at Check Browser Compatibility, Cross Platform Browser Test - Browsershots to make sure your overall page looks the way it should across ALL browsers and versions. It's just good to be fully aware.

STEP 3:
Get a trial setup with Crazy Egg and install their code on your upsell page(s) and deeply analyze the confetti map and heat map to see what the actual user activity is like on your upsell page.

STEP 4:
Check Google Analytics, or whatever tracking you're using, to see what the average duration is on your upsell page(s) to see if people are leaving immediately or what's happening. If they're leaving quickly, then use an optin pop that has "Exit Intent" Technology to try and save them. This is unreal technology. Don't use BounceExchange.com because they are a few grand a month. Just install Optin Monster if you're using WP and you'll be good to go.

I could keep going on and on and on and on and on Tom, but hopefully these 4 quick steps can help identify what's happening and give you some good insights.

Good Luck!

Thanks for this Chris, I'll get on it immediately