94 XLT with the 4ALD automatic with 185K, but completely rebuilt tranny about 10K ago. Lately strange things with torque converter lockup. Itusually works normally, with lockup right at 45 mph, and in and out like it should depending on load. If I stay under 55 or so, it is fine all day. If I get on the highway and cruise at 70 or so, it unlocks after about four or five miles, and then won't lock again - period, even if at lower speeds (but over 45). After a long shutdown, it again worked fine. This happened daily, whether warmed up to normal or not, but refused to do it for the tranny shop fellows after they fought with it for a whole day. So I took it home and it's doing it again. Lately, sometimes it doesn't lock at all when starting out in the morning (cold Michigan winter), so it's become less predictable and more intermittent. I assumed it might be something electrical (bad connector, etc.) but now it seems to be something sticking in the clutch activation mechanics. Do you think an additive might help, and then a drain, filter, etc. thereafter might help? Or is there something specific I should be looking for? Thanks.