Slow texting with AT&T

I guess my subject line is rather confusing. I meant I have T-Mo and the only person I text has AT&T.

I've had T-mobile for a couple years now and it's never really been TOO much of a problem to text. However, there have been a couple days within those couple years that texts would get seriously lagged behind especially at night. It never lasted more than one night or so. Recently though, it seems every night that it takes 20+ minutes to get a text to my best friend (or to receive one). Either way we both reply instantly and it takes at least 20+ minutes after 5pm or so.

My post is to see if there's anything I can do to maybe improve this. I called customer support and while pleasant, she was unhelpful. I ONLY text my friend who uses AT&T. Is there a number I can change or set or anything? I leave my phone in one place so it has service (albeit very little). This has been going on for a week straight and is not the text message support I am comfortable paying for as it is virtually unusable.

Any help appreciated and welcome.

Well I've always only had 1 or 2 bars and it's never been a problem before. In the last week though, my phone has kept trying to switch to Suncom and so I set it to t-mobile and changed it to manual so now it just asks if I want to. This is also when the problem started.