I am having the same issue with Straight Talk. I brought over 3 iPhones. (2 iPhone's 4 and 1 iPhone 4S). The data work great the first month, but after about a month that data became very, very flaky on all 3 phones. All 3 are unlocked by AT&T. I guess the old saying goes, Buyer Beware. I've called Straight Talk a half of dozen times, each time they walk me through the same thing. They check my settings (which are always correct). After that doesn't work, they tell me that their are service issues in my area. I am now going on 3 months of "service issues in my area" with the data.
I'm about to give up! After doing a search on the web, there seems to be issue with the "bring your own phone" plan. From reading all the posts, it seems that AT&T wants to limit how many phones Straight Talk can add. AT&T doesn't turn off the data on to the straight talk phones, instead they allow the data to be flaky enough, you will never have consistent data service. As I came from AT&T, I know it's not my phones or the service in my area. I never had an issue with AT&T until I switched to Straight Talk. Seems like Straight Talk forgot to tell their customers this. If all you need is Voice and SMS Texting, this plan is great but if you need data even a little, I wouldn't go with Straight Talk.
If anyone at Straight Talk is reading this and cares to keep 3 accounts, please call me at 208.201.5769 I'm planning on leaving by Sept 4th if I don't hear from someone who can actually help me. Going through my settings on my phone for the 12th time is not going to help.