Can't upgrade past 10.4-STABLE (interrupt storm?)

Hello,
I've got an older laptop that I attempted to install 12 on w/o success.
Well, it installed. But was unusable. Typing anything at the console
frequently doesn't output on the screen w/o tapping one of the arrow
keys. But doing that causes other problems. As I can't really use the
output. :(
I suspected an interrupt storm of some type. But really can't say for
sure. a vmstat -i seems to show unusually high numbers for irq1: atkbd0
often ~1/3rd the number for CPU0. I can easily realize all this during
the install process from the install media. So it's easy to test.
This is a i386 based Pentium M. FreeBSD 10.4-STABLE runs like a dream.
But I'm going to need to move forward at some point, and I'd like that
some point to be now. :)
Any thoughts on how I might discover /what/ change was made from
10.4-->11* to cause this?
It has a trackpad. Should that make any difference.

Thanks!

--Chris