Komatsu D21 P 6 left track not always turning

[poky1234]

Hello Everyone,
Need some advice on my dozer. I have just replaced the steering clutches and throw out bearings on both sides. Also replaced the transmission shift cable
Ran it for a short time. Dozer moved forward and reverse turned left and right but after a few minutes it felt like it was labouring to move. Jacked it up off the ground then ran it again. Right track moved forward and back but left track did not move. Backed brake off but didn't change anything. Left track looked tight so we backed it off . After that the left track staarted moving forward and reverse.
Do es anyone know what this issue might be?
Thanks Poky1234

 

[poky1234]

Does anyone have any ideas regarding this issue? The sprocket had a bit of play so I have since replaced the bearings in the final drive sprocket and the bottom roller closest to the sprocket. Still no change. Brakes are backed right off so thats not an issue. Idler spins freely. Dozer moves forward and back for a short time ,then left track stops turning and engine labours.
Thanks for any advice.