[TR] Overdrive intermittent

[scotts junk]

I recently had a chance to take a friend's TR-250 on a fairly long road trip
(1600 miles). The first day out the overdive operated in a rather strange
fashion: for the first few miles while departing the city it changed up and
down with the switch as normal, however once we hit the highway, it stopped
responding to any changes in the switch position (either up or down) BUT did
shift up into overdrive on its own at about 3800 rpm in third and stayed there
until the car was downshifted into second (at low speed) - almost like an
automatic transmission??!. Switch position did not seem to have any effect on
when it up or downshifted. It operated this way until our first stop (car had
the opportunity to cool down for about an hour). Again leaving town it
operated normally for a short time, but on the highway almost immediately went
back into "automatic" mode. It continued this throughout the first day (about
350 miles across fairly flat prairie, temp about 20C/68F). Next morning and
for the remainder of the trip it functioned prefectly normally. Has anyone
experienced anything similar or have an enlightening insight?

cheers
Scott