Toyota Repair: transmission down shifts on its own, throttle position sensor, sensor tps

Question
I have a 1993 previa van with 150,000 mile and an automatic transmission, every now and than while diving at speeds over 30 mpr it will down shift on its own for just a second and than go back to normal. Any ideas?

Answer
Although I have not seen this problem in a long time it could still be the cause, the problem is the wiring at the throttle position sensor,(TPS) the wires seperate inside the insulation and cause erratic shifting, the remedy is to replace the sub harness to the TPS, there is a repair kit available from the toyota parts department, it's listed as the TPS sub harness kit, it has to be spliced in to the existing wire harness.