I had the exact same problem with my 90 4runner. Problem was intermittent but on a long trip it would worry me and I seriously thought that I had a transmission problem as well. Then at some point the problem disappeared and never returned. I had some gaskets replaced for a small oil leak one day and they also replaced the thermostat while they were at it. I've never made the connection but perhaps it was the thermostat that also fixed my problem. Nice to have a chep repair once in a while, isn't it?