Is there any way to limit the number of units that can be run at the same time? at the moment it's downloading a load and running up to 32 at a time which is causing the machine to grind to a halt forcing me to reboot which then causes a lot of the tasks to fail.

I have tried the following config but no joy

<app_config>
<app_version>
<app_name>Theory</app_name>
<plan_class>vbox64_mt_mcore_theory</plan_class>
<avg_ncpus>16.0</avg_ncpus>
</app_version>
</app_config>

You can testing this parameter:
<max_concurrent>x</max_concurrent>

Just an example:

<app_config>
 <project_max_concurrent>3</project_max_concurrent>
 <app>
  <name>ATLAS</name>
  <max_concurrent>1</max_concurrent>
 </app>
 <app>
  <name>CMS</name>
  <max_concurrent>1</max_concurrent>
 </app>
 <app>
  <name>Theory</name>
  <max_concurrent>1</max_concurrent>
 </app>
 <app_version>
  <app_name>ATLAS</app_name>
  <plan_class>vbox64_mt_mcore_atlas</plan_class>
  <avg_ncpus>4</avg_ncpus>
  <cmdline>--memory_size_mb 4096 --nthreads 4</cmdline>
 </app_version>
 <app_version>
  <app_name>CMS</app_name>
  <plan_class>vbox64</plan_class>
  <cmdline>--memory_size_mb 2048 --nthreads 1</cmdline>
 </app_version>
 <app_version>
  <app_name>Theory</app_name>
  <plan_class>vbox64_theory</plan_class>
  <avg_ncpus>1</avg_ncpus>
  <cmdline>--memory_size_mb 768 --nthreads 1</cmdline>
 </app_version>
</app_config>

Be aware that app_config don't have influence how much tasks are downloaded from the server.
You have to reduce that with the buffer setting with BOINC Manager.

Cheers, that will teach me to troubleshoot before caffeine, ill see how I get along with it.
Does anyone know why loading up 32 VM Theory tasks lags out the PC? I saw an old thread from 2020 about it but couldn't see a resolution.

The Scheduler from Theory have the default .xml

My system also was grinding to halt with virtualbox.
I had to use vmware workstation linux vm to run lhcathome.