when adjusting (tuning) the rear derailluer on one of my many throw-together bikes- i noticed cable tension has to be at one extreme to shift up, and the opposite extreme to downshift... but when i center the barrel adjuster, the derailleur skips gears both ways... then after an attempt to adjust to correct the skipping, it ends up at either high tension or extremely low tension.

Any suggestions to a fix?

What kind of shifter? Sounds like the indexing doesn't match with either the cassette or the derailleur.

i got the bike at a junkyard as a frame with the rear rim and derailleur, the rest is "throw-together". the shifter is from a shimano setup while the derailleur is sram... thats probably the reason of error.

A few possibilities come to mind. Considering the probable age and condition of the bike, it's likely that wear is degrading the performance.

The RD pivots might have developed some play, worsening response to cable inputs.
The housing could be worn or gummy with old grease, making cable return slow
The detent's (clicks) in the shifter might be worn throwing off the shift values.
The chain is worn and now has more side flex, making it resistant to the horizontal shifting force.

These are just for starters. Index systems wear and shift response degrades over time. On a bike you ride all the time you barely notice it and compensate by overshifting slightly or other ways until you decide to replace stuff. There are some things, like new cables and housings that might help, but there's no way an old system will shift like a new one no matter what you do.

the cables and housings i replaced last week. so i might have to replace the actual components?

I'm not 100% sure, but I do believe that most, if not all, SRAM derailleurs use a 1:1 cable pull/actuation ratio and Shimano have always used 1:2. So your shifter is pulling too much cable for your derailleur.

very possible. anyway to check or change it so i dont have to buy another part?

I hadn't noticed that you had Sram shifters and a Shimano RD, and that might be the problem, though it usually causes overshifting and poor spacing. Also, Sram makes many shifters specifically for Shimano. The rule is Sram shifters with numbers like 7.0, 9.0, or X.0, are for Sram systems, and those with names such as MRX, attack and rocket, are for Shimano.

So check that the shifter and Rd are compatible, then if it isn't a cable check both the RD and shifters for slop, and decide which, if any, to replace first. Given what this bike is, I'd go slow about sinking too much into replacing a string of components, but that's just me.

1990's huffy ironman (triathlon pro) frame the stickers on the frame make me assume that it came with ALL sram components.. i can get a few pics up of the components ina bit.

check for bent hanger

pics:
1st= curent RD
2nd= older RD
3rd= previous RD (broken)
4th= sram index shifting sticker
5th= brake/gripshift (one piece)