If your furnace has been working for several years, then the double 90 and back-slope should not be an issue. If they were, you'd have had these problems day 1.
The furnace shutting off could be due to the inducer pressure not being within specs due to the buildup of condensation while the furnace is running. Again if this is a new issue and not a problem since installation you know where to look for answers, the draft inducer assembly.
I would first check all hoses involved with the inducer assembly. Look for cracks, clogs or pinched places. Any of these can affect the pressure at the pressure switch.
Next I would disassemble, if possible, and clean the 2 condensate drain pipes. Again clogs, especially in any P trap, will affect performance and resulting pressure achieved by the inducer.
Next see if you can remove the inducer blower wheel and inspect it for debris like dust and other matter that could affect performance. Wiggle the motor shaft looking for excessive movenemt side to side, top to bottom indicating possible bearing problems.
Finally the pressure sensor itself could be defective. I would save replacing that to the very end. If you can remove it and inspect it, do so.
Unless your house has settled since the furnace was installed, the pipe pitch has not change. Since the furnace was working fine for years before now (I assume), the pitch should not be an issue.