Gears shift perfectly when suspended on rack, but not on the ground? So I had recently solved some issues I was having thanks to the community. I re-tuned my bike on a suspended mount and everytning looks great. On the ground, all of the gears work smoothly except for shifting into 8th gear (8 speed rear derailleur). The chain hangs on 7th and won’t jump. On my bike rack it shifts perfectly after properly setting the high limit screw. The only way I’ve gotten it to work on the ground is backing out the high limit screw more (small turns), but the shifting is sort of a jolt and it occasionally overthrows the chain and derails it (...bad joke). 

Any suggestions or tips on what to look for? Nothing appears bent, bolts are tightened, wheel is true, and bike is tuned. I’m seriously scratching my head. 

Thanks
While I'm not sure this qualifies as an answer, because with such questions there is usually a lot of guessing, your issue sounds like a flexing issue or a worn rear-hub. 

The flexing could happen because of your weight (maybe fatigued frame? or just flexible) resulting in slightly changed geometry while riding vs suspended mount.

A worn rear hub could cause a wobbling or light tilting in the cassette while under load.

I would also check the cables while the bike is mounted on the stand and see that the clamps or the bike position (handlebars sitting at a hard angle, pulling on cables) are not somehow affecting the tension in the derailleur cable.