Task Scheduler doesn't work

Question

Slightly leading title. It does work when you set the tasks up and works fine for a while. Then all the tasks begin failing at some random point.
I have several small EXEs and a piece of kit called SyncBackup in the server. These are the tasks to be executed.
Sticking with the EXEs I built myself which are .NET apps, these are lightweight apps which connect to a database and write to a log file.
RDP in as Administrator and double-click the EXE - runs fine.
Set up a Scheduled Task to run it every 5 minutes - fine. Right click, run task - executes perfectly. Job complete, last run status fine.
Go away and leave the server for a few days - runs fine.
Then it just stops working with code 0xC0000142. For every single Scheduled Task. Except one which I think returns 0xC01.
Task History shows "Action completed" and "Task Completed" as if it's all working perfectly. But it isn't. No errors logged or evident there.
I have these all set up to run as the Administrator for testing purposes.
Why do these tasks run fine when set up and break "behind your back" at some later stage? What does this error code mean? How do I get these working and importantly, *stay working*?
I've seen stacks of threads like this with no resolution. A common issue appears to be the Start In directory specification.
It doesn't make any difference whether you specify the Start in directory or not. There are no spaces in the file names or paths anyway so quote marks irrelevant.
And, these tasks do work initially - they just break later on.
I've come across a piece of third party kit which emulates what the Windows Task Scheduler does and am tempted to install and use that instead.
But the Windows Task Scheduler always used to work... ;)

Answer 1

I was waiting until this evening to see if restarting the server would fix this.
And it has.
All the tasks work perfectly - those failing with both the error codes, so that's my own apps and also the SyncBackup tasks.
For now...
Digging around some more, I'm led to believe this might have something to do with "Desktop Heap" size.
http://blogs.msdn.com/b/ntdebugging/archive/2007/01/04/desktop-heap-overview.aspx
Thing is, this never used to happen up until very recently on one of my servers, I migrated to a new one (not because of this) mystified by the problem and hoping the problem would vanish. And it did, at first.
One other server I have which runs SyncBackup tasks doesn't have this issue.
All Windows Server 2008 R2 and all up to date with hotfixes.
I'm reluctant to modify the Desktop Heap size without a more detailed understanding and still mystified as to why this all worked perfectly until about three weeks ago.