It happens. It could possibly just be your iPhone. I've always had problems with reception at my work place in certain corners at all times. In my house it's perfectly fine. I've run my iPhone on turbo sims, yellowsn0w, then ultrasn0w now and it has always been the same reception (except with the darn turbo sims, those things just make iPhones suck reception-wise 50-60% of the time).
The internal antenna is on the bottom of your phone so if you're low on reception make sure your hand isn't covering the bottom part of the phone (where you charge it) and that might help a little. Hope this helps a little.
This has been discussed in many threads. It may be a problem with the jailbreak process. occasional something get corrupted.
If this continues to be a problem, my advice would be restore in iTunes to a fresh copy of 3.0. then re-jailbreak with redsnow and unlock with ultrasn0w.
Good Luck.
It seems to be when 3G is turned on that it really drops so im thinking its maby just my house its fine when im outside and have 3G on but when i turn 3G off in the house the signal seems to stay up
I will leave it and see what happens then if it gets bad i will restore and re jailbreak and unlock