weird vacuum at idle?

Hey, i have a redtop sr20 in my 92 240.. and i'm having a weird vacuum problem at idle. sometimes it idles with 10 mm/hg and sometimes it is 15 mm/hg. i know it should be around 20 mm/hg, and i'm really not sure why it fluctuates sometimes. if i rev it above 1k rpms, the vacuum starts to go more towards 20 mm/hg, so i'm really not sure what this could mean. i've checked the timing, and it seems good... the next thing on my list is to build a boost leak tester, but i was just wondering if anyone else had any ideas on what this could be.

i can hit 7psi easily by the way, just to let whoever know, in case that would help

thanks