Came here looking for advice on a Trane XV90. Didn't find a direct answer but I think my problem is resolved now: one of the pressure switches failing, causing short cycle of heating operation interrupted every 5 minutes or so. The failure seems to be on account of condensation from the high efficiency heat exchanger, building up to the point that it enters the pressure switch. I heard a gurgling noise during operation and then when testing, pulled the switches off the bracket and found the lower switch had an ounce of rusty water in it! At this point it is completely non functional.
On this XV90 model the pressure bib for the back sides of the two switches is located very close to the condensate drain. I think if the drain were blocked (as it was at one time) the rising water level could cover the pressure bib, allowing entry through the tube. This explains the presence of the water inside the switch.