Yeah you're right about thinking it would happen in all other gears as well.
I checked my transmission fluid levels and they're perfectly fine.
I'm not sure what was done during the flush.
I thought it was a standard procedure, like an oil change.
It was suggested to me that it might be the speed sensor on the transmission because it seems to me, while I'm driving, that the car's confused as to which gear it should be in.
As I'm speeding up slowly (from a stop to about 35mph) the transmission is constantly trying to shift into the highest gear.
Once it realizes that the gear is too high I get that "rumble-strip" sound and it down-shifts, after which it tries again to go to the highest gear, until I'm going fast enough that even in its highest gear, the rpm's are above 1500 and then everything's fine.
So from my observations, this only happens at slow speeds.
What confuses me the most is that, if I shift into the auto-stick mode, this doesn't happen at all. Of course, I decide when to up-shift, so that might explain that, but still.
I don't get this at all.
I'll have to have it looked at sometime next week I guess.
Thanks for the feedback guys