HP plotter T790 Image quality prolem

My quality images intermittently become bad but I don;t know the cause or causes of this quality problem(my machine HP T790 plotter)
I guess because of following causes but please introduce me about troubleshooting this problem.
1- cloging and entering air into the plotter heads(vaccum problems)
2- use old charge ink bottle for more than 2 years
3- misconfiguring paper weight 170 gram china coated paper into heavy coated paper on the panel or because of bad paper quality
4- other causes

thanks a lot for your suggestion and solutions described
please introduce and explain your solutions

 

I'm experiencing similar issues I think.
I'll share more information later, I'd love to know how to fix this.