Multiple errors, common problem

Hello

I have a mid 2010 iMac, running OS X Yosemite 10.10.5 with 16GB RAM, 1TB HDD

I have 4 symptoms that now seem to arise together from time to time. Timeline may be connected to the latest security update in July.

i) Google Chrome crashes out on startup by failing to load extensions and showing broken tabs, or new tabs fail.

Unistalling and reinstalling latest version of Chrome (even Chrome Canary made no difference)

ii) terminal fails to load showing [forkpty: Resource temporarily unavailable] [Could not create a new process and open a pseudo-tty.])

iii) dmg's cannot install, with system declaring "No such file or directory".

iv) About this Mac pane loads empty or populates very slowly.

Symptoms usually disappear with restart, and then return intermittently.

Scans by Avira, Malwarebytes and Sophos do not show any malware.

System disk and permissions do not show anything significant. Internet searches imply the problems appear in more recent OS X releases too.

Any ideas, what to check next.

Thanks