r100 trans issues

[tatush3]

i have a 2001 superduty dually,207,000 miles,pulling a 2006 toyhauler 40 ft, i had trans rebuilt with billet converter (3600.00) 4000 miles later reverse is shuttering, sometimes none at all,i did filter and fluid,no change,it does have a powerchips program tow safe mode,i installed stock program no difference,the company wts in kent,wa is no longer in buisness,ie no warranty any suggestions) I really dont wanna buy another trans,but i do need my truck