My 1995 Suburban C1500 appears to downshift from OD to 3rd and back again at highway speeds. This never happens around town, but only on the highway after TC lockup. Sometimes it happens once and not again, sometimes it happens repeatedly anywhere from twice in 3 minutes to randomly spaced occurrences over a longer time. The TPS has been checked and tests OK. The EGR was causing an SES light that seemed to be related (the light came on at highway speed and went out when the downshift occurred, but came back on again 10-15 minutes later). The EGR was replaced and the SES light is out, but the downshift still occurs. No codes are evident when scanned by the repair shop. Any body have some ideas?