I have a trimatic behind a 308 which for short trips (under half an hour- which is most of the time) works fine, shifts well, engages straight away. But after longer trips involving freeway speeds and when I got off freeway and come to rest at traffic lights feels like it won't take off or takes off slowly, slipping basically, then engages after maybe 5 seconds with a shudder and I'm away and shifts normal up to 2nd and 3rd without any noticeable slipping.

I checked the oil level of course, but not sure if I've got it right. It was showing solid fluid to the "add" mark and then fluid on side half of the stick to the "full" mark. I presumed it really was only at the "add" level so added half a litre, but symptoms still the same if not worse (although worse after a longer drive than its done before). Now oil is showing solid to "Full' on the dipstick and on the side half of the stick another inch above Full.

Wondering whether I've now overfilled it and I'm aerating the fluid at speed? or is it a torque converter on the way out? or something else?