The servo only applies the band for 2nd, & 4th.. If it is slipping in any gear other than 2nd, or 4th, then no servo on the planet is going to help.
If the trans has had a hard life-it is probably time for an over-haul. Once they start slipping, they don't last much longer after that.
Here's another idea that I think you already hit on-don't drive it under full throttle at least until you have the dough to shell out for a new transmission.