P1815 is kind of a "plausibility" code for the Manual Mode switches, like the TCM seeing two gears at the same time, etc. The chart on AT-94 defines it as, "When an impossible pattern of switch signals is detected." It's usually electrical and not a problem with the tranny per se, but since the TCM on your trans is built into the valve body, your VB or TCM might have a problem. I can't get into ASIST right now to see if the DTC has a failsafe condition, but there's a good chance it does and your laggy/non shifting is a result of limping in failsafe. It's possible the TCM just had an electrical glitch, so I'd try simply erasing it to see if things improve and whether it re-occurs.