If boinc is running you can request more work from GPUgrid by executing the following command in windows or linux

C:\Program Files\BOINC>boinccmd --project www.gpugrid.net update

Perhaps you can incorporate this command into a shell script and set to run using Cron every 5 minutes or so?

Maybe using windows scheduler too?

The solution might involve setting some projects to Resource Share 0 (such that they are only asked as a last resort, and are backup projects), while also setting both cache settings to 0 (such that the non-backup-project is frequently asked for work).

Alternatively, if you want it to ask for work often while also maintaining a cache, then set the Minimum Work Buffer ("Fetch at least") to a cache value (like 5 days), but make sure your Max Additional Work Buffer ("But not much more") cache value is 0. Then it'll try it's best to keep that 5-day-water-mark, asking the project for work frequently to keep it floating at that level.

Is the computer connected to multiple projects?
Are you trying to make it only work on a single project?
Are you wanting it to check for work as soon as it's near completion of the last task, and then only fetch 1 additional task?

I can probably help you further, if you give us more detail on your scenario.