Dropdowns become erratic when logged in

I just installed the .dev version to take advantage of active trail for 2 and 3 level menus on my sites. Tweaked the CSS to get that working, so that's fine. Previously I've used nice menus for well over a year on over 20 sites and all has worked well. No prior problems like this or otherwise.

When not logged in, the new menus seem to work fine at all levels. Everything is visible and works properly. You hover over a menuparent and the children appear as you'd expect. You click on a child and active trail works nicely. So, when not logged in, it works the same as before - except you see active trail styling. Tested and confirmed this on multiple sites (all with nearly identical menus) in latest FF, Chrome and IE (all localhost based - live sites still on prior version).

Problems arise, though, when logged in -- the menus behave very erratically. I've tried on multiple sites and in IE, FF, and Chrome - same result each time. Hard to describe what happens, but the basic idea is that when you mouseover the top level menuparents, dropdowns don't appear solid and steady. They sort of flash in and out -- not in any steady repetitive pattern, but randomly they might appear for a split second as you move teh moue around over the menuparent. They appear a little more reliably when you mouse off the menuparent. For instance, you slide down the menuparent to where the children should be and they sort of appear there for a second or two before disappearing. In those few seconds, you think you can click on them, but doesn't take. Menu link doesn't activate.

The top level menu items themselves appear as normal and these menu items are clickable letting you move from page to page as normal. So this only affects the dropdowns.

I've made very few changes to CSS from before to make active trail work, so hard to imagine that's the cause (plus the CSS definitely works when not logged in). I looked at z-index thinking maybe that, but behavior is too erratic for that.

I have no nice_menu overrides. JS option is checked. Ran update.php and it showed nice menus updated with no errors. Same module lineup.

I have cleared cache in multiple ways - including directly in cache_menu table via phpMyAdmin. Closed browsers, cleared their cache. Doesn't help, not even for a few seconds. I have noticed that the cache_menu table seems to get relatively large (at least as compared to my site DBs). Tables quickly grow to 1M+ -- but that could have been happening before. I'm not sure.

Any ideas what is causing this? If not, I will go back to prior version. Shame cause I like the menu trails.