Toro Time Cutter Making Ugly Tracks in Lawn

I have a Toro model 74633 Time-cutter zero turn mower.  The mower has the Kohler V twin motor.
The motor is cutting out under a heavy load, I can sort that out. The problem I need help with is the mower makes tire tracks at times but not all the time. The tracks, when they occur look like 1 rear wheel is either slow and being dragged along or it may be spinning, at times.  I mowed today, my yard is about an acre, the mower made several long tracks, looks like the grass has been scuffed and heavily damaged. These tracks will last at least a week or more before the grass grows back.  The lawn is Bermuda grass in good condition.   I am studying the manual for hints.  I wonder if a hydro is malfunctioning or the brake is grabbing. I do not feel anything different when the tracks are made.

Tire pressure?? The manual says   12psi the tire is marked 24 psi. I will go by the manual.
These tracks do look like skid marks and do not occur  in all the lawn and appear only in straight runs. I mow fairly slow to get a good cut and this this happens.   How does the brake work?? U think the left wheel makes the track.  Could the brake be hold ing the right wheel making the left wheel slip from pushing harder?   Some of the tracks are 30 feet long. My lawn looks terrible. 
 

I have always felt that this mower was a little "twitchy"  at times and sometimes I think the steering is not as responsive as it could be.  The mower seems jerky when backing out of the shed and does not turn well on a side slope, just does not turn as I think it should.  Check / adjust the linkage on the  levers??
I can not tell if the tracks are from spinning, slipping or whatever. 

An update on this item.  New spark plugs, adjust valve lash, fuel filter and Sea Foam in gas.  Also adjust deck/level deck, some more.  Rear tires are marked 24psi, manual says 12psu, local dealer says 6psi. I compromised to 10 psi, front and rear tires. The motor runs really well and it mowed the acre without a misfire. I did not see any "ugly tracks" this time.  Lawn looks good, still waiting for the "ugly tracks" to heal.  You know what they say "it's the little things that matter".  Thanks for all the input, it is appreciated.