My van has been surging?

For about a year now my van has been surging. It doesn't matter if you are driving 25mph or 60mph or if you are using the cruise control or not. When driving you can feel the van lunge forward and the RPM's will jump about 200. Not alot but enough to notice. I took it to my mechanic last summer and of course it wouldn't do it for them and there are no codes in the computer. I also took it to a transmission specialist and they didn't know what was going on either. The fluid is a little brown but both shops suggested not to change the fluid because if it is the trans it could make it worse. Around the time that the weather got cold I had to change my alternator and the surging stopped. I thought the problem was solved. But as soon as it got hot and humid it started back up again and now it is worse than before. It seems like the hotter and/or wetter the weather the more it acts up. Any ideas/suggestions? Thanks!