Almost the same exact thing happens to me. Except that after I connect to WiFi for a few hours, not only do I lose my ability to use 3G, I lose the ability to do everything. No calls, no text, nothing.
I bought this phone on Friday. Went home, and spent nearly two hours getting it activated and working. I called customer support and they got me up and running. And it worked fine. Until I connected to my home's WiFi. The next morning, the phone would not respond to the *22890 number. It would say it was programmed, but I would get an 'Insufficient Funds' message when I tried to call anyone. So, I had to call customer support again -- another 30 minutes spent doing this. (At this point, I just figured something was a little off with my phone.)
I finally got it back up yesterday morning. It worked fine all day long. And then I got home, and again, connected to WiFi. When I logged off of WiFi about five hours later, I'd lost the ability to call or text or get 3G service. Again. And *22890 has really done absolutely nothing for me, since it says it's programmed, but I keep getting the same no funds message. There's another number that CS gives me when I call, but I cannot remember it at the moment.
And now I've got to call customer support. Again.
I had *hoped* that I could save myself (And StraightTalk) some data usage by connecting to Wi-Fi when I was at home. I thought I was being a good customer. But I'm starting to see that unless they have a fix for this, I'm going to have to avoid Wi-Fi like the plague. I don't use a ton of data anyway, but, geez. Talk about shooting yourself in the foot, StraightTalk.