Ok with 13k miles or 22k kms doubt that it would be a tranny issue. But since u said it gets worse when hot then it may be fluids. Start by checking the level of tranny fluid and also check your differential level. A shudder can also be the rear end low on fluid. Also check to see if the fluid is burnt or has a burnt smell to it as well as metal flaking in the fluids
1996 Mustang GT, 17x9 front and 17x10.5 cobra r replica rims, 255/40/17 front 285/40/17 rear at the moment,
o/r x-pipe flow master 40 series dump downs, cai/ram intake, 75mm c&l throttle body and plenum, pi heads and intake swap, msd wires, stock gears, untuned at the moment, no dyno numbers for now.