Signal bars fluctuating

[allanes5]

I've been noticing on a consistent basis that my iphone 4 signals keep fluctuating.

I get 2-3 bars in one spot and sometimes it goes down to one till I eventually lose all and my phone stops working. Can't make or receive calls.

I try to move to areas that I know have high signal strength and the phone stays at nothing still can't make or receive calls. I put my phone on airplane mode and then take airplane mode off and I get a "sim failure" message.

So I turn the phone off and then on and I get 4-5 signal bars where I used to get 2-3.

This happens in many places. Anybody have an idea what's up with this???

Edit: I'm on 4.0.1 jailbroken with ultrasn0w