I suspect it started after latest (1.31.405.4) ROM installation and happens periodically - you just tap the contact and nothing happens (sometimes at all but sometimes it reacts after minute or so and suddenly starts to call).
There could be at least two causes:
1. No reaction on actual "tap" (hard to explain why it finally calls sometimes but I noticed that Call button could give different reactions comparing to clicks on actual contact)
2. Process that executes call is doing something in the background (freeing up memory? restarting? doing some sync?)
After it "breaks through" it works for some time seamlessly - you just touch the contact and it calls immediately but after some time (even after phone reboot) the glitch is back
NB. I uninstalled almost all apps (including beautiful Timeriffic) I've got from Android Market and prey it's not related to the Exchange Sync and apps around it... Let's see if I still have problem tomorrow.