So, my issue is, my tasks are going exceedingly long. They are supposed to last 3:19 minutes according to BOINC manager, but when reaching 80% completion, they start to slow down. As a result, the tasks last even as long as 30 minutes.

Project I help is FiND@Home. My CPU is intel i-7 2600, it uses 80% of processors and 70% of CPU time. GPU is Nvidia GTX 560 ti.

Is it possible to fix it?

BOINC does not have control over how fast or slow a task runs. That is application/project specific and also depends on what else is running on that computer besides BOINC. That said BOINC, is usually NOT VERY GOOD at estimating run times.

A project may hard-code a specific estimated run-time in hours/minutes, but that is a very rare practice and estimated run-time it is usually specified in FLOP COUNTS.

It looks like you are NOT the only one seeing this problem:
FIND@home Message boards : Number crunching : Latest workunits have bad runtime esrimates

That is good to know! Thanks! Come to think of it, I should dust off me PC, since the temperaturess Boinc puts it through, do not herald a long lifespan.