That still sounds from your test that there is obstruction of the drain ports. I'm guessing those square holes in the frame bottom are your ports. If so, you might use an eye dropper, syringe, or some other device to flood each port individually and check how well they drain.
Bugs can crawl a long way into a window drain system and die, partially plugging it, as I can attest from the tiny bees that got into our upstairs vinyl windows. They had to mosey several feet and negotiate under the bottom slider guide to get in. I was advised to put small amounts of cotton in the interior drain ports of all the house windows. That kept the bees out of the house and let moisture drain, but didn't necessarily keep them from plugging things under the guide.
If one or more ports don't drain well, you might fashion a wire you could feed through it and try dislodging any obstruction through to an outside weep or another inline port where you can remove it. Beyond finding such a plug, I reckon Bretz is your next best option, Jerry.
