Except for changing the switch time "Write to disk every Switch between applications every xx minutes" to a value higher than the longest running task, there's no way to force **. BOINC balances the project shares over time about every minute [WCG is one project]. When a task checkpoints, the client looks at who's got highest scheduling priority, and if it's a different project, then it will switch at checkpoint save. Then, if any task is under deadline threat [advanced cerebral cortex understanding required], the task will be started no matter what, in order of earliest deadline first. Those tasks will run to the end, unless more projects have tasks with deadline threats... all processing then with the description "Running, high priority".

With multiple grids attached to a client and active and each having different deadlines and runtimes, that's what to expect, in a nutshell, SAN.

edit: ** Well there is the micromanaging way, manually suspending and releasing tasks, which often makes matters worse, plus, if a task of a project [any of WCG e.g.], then the work fetch is suspended for that project [Maybe not a bad thing if tasks are under deadline threat] ;>)

edit: Brain-Fingers disconnnect typo [see post Scribe 3rd from top]

....and the other thing I would do is set the "Switch between applications every" setting to something like 10,000 minutes which should mean you finish a WU before swapping applications.

If this will not help, upgrade BOINC to version 7.0.xx. AFAIK, the problem you are mentioning used to affect users with BOINC version 6.xx.yy

Cheers

ETA: is it something similar to David Autumns' problem we discussed here?

v6.10.xx has seriously screwed-up handling of deadlines and high priority. For the early versions like 6.10.1x - v6.10.2x you could easily have 100+ partially-crunched tasks for the same project, but thankfully this was atleast improved in the later clients.

But, another bug never fixed in v6.10.xx is, even if a task was in deadline-trouble, and even if the deadline was passed, the client could instead choose to run work for another BOINC-project with 1+ month until the deadline and with no deadline-problems.