Problems in Timing Belt drive system in 6WD robot

Sorry for the cross-posting,

I'm having problems with the timing belt drive system in a 6WD robot I'm
building for Robomagellan competition in the upcoming Robogames.
(Mechanical design courtesy DPA).

Pictures in LM forum:

http://www.lynxmotion.net/viewtopic.php?f=9&t=7839&hilit=6WD&start=45

There are a pair of DC servo motors, each driving a short (3") 037
(3/8") .2" pitch timing belt from the 2nd level down to the first, 23:15
ratio for a 1.33 speed increase. The motors are mounted on opposite ends

On the first level are three short axles on each side of the 'bot,
connected on 8" centers by the same timing belts, and 20 tooth pulleys.
There are belt tensioners (4) between each pair of pulleys pressing
down on the top of the belt. (I do remember hearing that the tensioner
pulleys should be on the driven side of the pulley, so one pair is on
the wrong side).

The belt system appears to run fine in straight runs, grass, concrete,
etc., even when moving quite fast. The problem arises when the 'bot
starts to make a turn (more torque required). I start hearing loud
"popping" sounds, which I take to be the belt(s) jumping teeth.

I can't determine which belt(s) is jumping. Since I first observed
(heard) the problem, I've strengthened up the bearing supports between
the axles, made a better tensioning nut for the belt tensioners
(idlers), and increased the tension on the upper drive belts as well.
Specs say 3-4 oz at 3/64" deflection on the belts, although I don't have
a way to accurately measure it. The belts "sing" when they run, like
they are being "plucked", so I think they're pretty tight.

The angle of the upper motor belt is about 50 degrees, and I haven't yet
figured out a way to add a tensioner to it.

Any ideas?

Thanks!

Alan KM6VV