I've tested with all CC and hacks removed and I still get the same problem.

When I test in another neighbourhood (Belladonna Cove was the one I tested in) with all CC and hacks included, I don't get the same problem.

I've been playing this hood since July 08 with all the same hacks and CC and it's only been since I tried to play the lot that freezes before the carpool and schoolbus that I've had a problem.

The only common element I can think of is that in two of the lots, two sims have the same CC hair.  However, in saying that, they've both had that hair since they were toddlers, one is a teen and one is half way through her adulthood so its not a new thing.

I agree, though, it's definitely sounding a lot more general than I first thought.  Damn it.

For clarification:
Lot one: game freezes at 12.59 the carpool and the schoolbus are due at 1pm
Lot two: game freezes at 2.59pm.  Everyone is home and nothing special is due to happen at 3pm
Lot three: game freezes at 5.59pm.  Again, everyone is home and nothing special is due to happen at 6pm

So although I assumed that the freezing issue was related to the carpool and schoolbus, it seems from further testing that it is really just quite random!

I've left the game at the freeze state, waiting for the schoolbus or the carpool, for over half an hour before turning the computer off at the switch so I'm going to assume that it doesn't eventually come out of the freeze state.  I've not left the other lots this long.

I experimented with moving the family from lot two.  I could move them into the bin just fine.  I loaded up a randomly created placeholder sim in a small lot and using the teleporter bush, I teleported the family into that lot.  Using the bush again, I then moved them all in.  I could save the lot just fine and re-load it without issue.  The lot is tiny so it's not really suitable to just use as is.  However, when I tried to move them into a new lot with a Maxis made house, I get the family loading screen, the cursor changes from the arrow to the hourglass as it slowly turns around, so it appears to be loading normally, but it doesn't ever get past the loading screen.  I tried leaving that for a good hour or so but still nothing.

The one thing I haven't tried loading them into an empty lot and building a house for them.  If I have this many lots freezing randomly, doesn't seem like it would be worth it!

Looks like I might have a bigger problem than I first thought.  Out of the past four lots that I've tried to play in that neighbourhood, three have had the freezing problem.  The other two just froze at a random point (but again, one minute before the hour) when nothing of significance was supposed to happen.  I moved one family out and now am unable to load them into a new lot.

I did mange to do the force error on the lot I orignally posted about and got errors on so many things it was ridiculous.  Everything from the windows to the sims wedding ring so something called "Emitter - Smell Negative".

Although it would be good to know what on earth has caused this, I'm at the stage now where I figure my hood is pretty much borked.  I did some play testing of a Maxis neighbourhood and it appears to work just fine.

Thank you so much for your help, magicmoon, I've learnt an awful lot just from your responses.

But in case anyone else has a similar problem, I'll include answers to the questions that you asked although they're pretty much moot to me.
- Career was the Entertainer career that came with one of the EPs.  I don't have any custom careers at all.
- The carpool does behave properly on other lots
- The nuke menu did nuke a few things but not enough apparently.

I put testingcheatsenabled on and no error message popped up to be able to attach a log for.  However, this time the carpool did actually arrive in that it was on the lot, it just hadn't quite got to the spot where it parks and beeps annoyingly.

I'm using all EPs and all Stuff Packs.  All hacks and downloads that I have in my game have worked fine up until this point and I've not downloaded anything new in months and months. (I've already tried taking out all CC which didn't change everything - the error is being very persistent)

I have a household of five sims.  Last time I saved, one was at work, three were at school and one was at home waiting for her carpool.  Her carpool is due at 1pm, same time as one of the kids is due home from school.  This lot had started to freeze at 12.59.

I can force an error using the batbox at any time before 1pm and they all come home but the problem occurs again when any carpool or school bus is due to arrive.  I've checked out my portals and they're all there, same as any bog standard lot.

I've been playing this lot for quite some time now and this has only just started to happen.

Any ideas as to how I can fix this will be greatfully received.

Thanks.