I feel for you! Is the locos you have Spectrum models? I have a few of them myself, including the 2-8-0 and a 4-6-0, and I have no problems with them. They manage to pull 10 NMRA weighted 50´ cars up my layouts 2% grade (including 18" curve). I run my on DCC and have not seen the maentioned bogs or speed reductions. Sorry that I don´t have any solutions for your problems.
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Buying Bachmann loco without trying it - there is fair chance you will not be satisfied, it seams.
Beeing from Europe, I do not have that option, so my two Bachmann locos were a bit of gambles. Shay ended up being great runner, and 2-8-0 (with bad Bachmann decoder) also run good. Maybe I had a bit of luck.
I sold Consolidation later because I have 18' min. radius, and this loco likes straight track and generous curves. But Shay is one of my favourite models!
Spectrum 4-6-0 looks like a nice engine, so maybe one day...
Note to self: don't get any more Bachmann HO steam if I intend to have it pull more than 4-6 cars.
In an earlier post, I mentioned that the two 2-8-0's and the two Decapods I had both ran poorly at low speed, slowed down badly on grades, and could pull only a few cars up a 2.5% grade. This was with either DC before conversion or DCC. I did all the usual and suggested things to make sure there was no binding, etc. My conclusion was that either the gear ratio needed to be higher, the motor torque higher, or both. I decided maybe this was so because these were small locos with small motors, and not very good mechanical designs. Some people agreed and some not.
Last weekend, I saw a Bachmann Santa Fe 3781 (a large Northern) at a train show and took it home. This thing acts just about the same. It stumbles at low speed. It slows down on 38" curves, which seems unbelievable, since I verified free lateral movement of the driver axles and rods. It doesn't like grades, even running light. And ten free-rolling freight cars bog it down pretty badly, such that the chuff calibration of a sound decoder I put in it is thrown way off from a "light" calibration. Forget about grades with more than six or seven cars. This engine has been carefully cleaned, checked and lubed.
Not that other steam I have (proto 2000, BLI) doesn't suffer some of these problems, but my Bachmanns seem the worst. Too bad, because some, especially the Spectrums, are quite good looking.
Hal