This does sound like it could be kernel related. Seeing how I didn't really modify to much in the ROM and it has ran reasonably well for lots of people the few that are reported having issues with lockups should consider using different kernels. Plus many of the kernels now are set to run at 100-1000 mhz, my device doesn't handle 100 mhz very well and a few weeks ago prior to even building ACE ROM v2.0 I started running my kernels at 200-1000 mhz and it is alot more solid/dependable now.
I would start by bumping that bottom number on the CPU settings to 200 instead of the 100 mhz and see how things go, it may resolve things for many as some phone just don't like waking up or idling at 100 mhz..
From there shop around and try the different kernels available,
Twilight/Vision/Genocide/Baked Snack or even pull the Bonsai kernel and flash that if your more tech savy with those type of things..
I'm not saying ACE is perfect as no ROM really is but there is a small portion of users reporting more than "average" lockups or FC's than should be happening. Plus seeing how its so random and not isolated to any one spot/app that leads me to believe even more that it could be kernel related.
Only 1 reported area, the occasional lockups on shutdown I can confirm this has happened to me and a few others otherwise eveything else has been very random and sporadic.
Hang in there, try experimenting a bit and you may surprise yourself..