Well your throttle cable is a huge part to manual tuning without it being in the right condition at at the right exact point it needs I would just say replace the cable, but since im not quite certain as to what your saying; Is it the grommet actually connected to the firewall that the cable just runs through or a grommet that actually goes around the cable that stays in a certain place so that when the accelerator is pushed the cable runs freely through that grommet but stays at the right angle and height so theres no excess pull on the cable causing a different throttle rate. If it is one that is on the cable I would say just replace the unit, unless you know exactly where the grommet is supposed to go on the cable and the cable doesn't look bent or kinked anywhere then just go to a home depot, lowes, etc... and try to find one that is relatively similar and will hold the cable but not pinch it too tightly.
If its the grommet on the firewall, than just go get a new one at home depot, lowes, napa..etc... as far as the cable coming loose because of a missing grommet and having to be re-adjusted often (unless your trying to find the sweet spot when manual tuning) than I dont know how that would happen unless its just unhooked from a cable harness bracket somewhere.
Anyways sorry i'm not much help but tell me what happens when you are done figuring this problem out I might need to know it in the future. goodluck.