I have been battling this problem for some time too. I have a 3G and 3GS, both jailbroken and both has sbsettings installed. During one of the trips, the 3G has good GPS signal thoughout and the 3GS kept alternating between colored and grey map.
I then did a full restore on the 3GS, and everything works perfectly once again.
After a few days, it became cranky once again, and as you know it is a big exercise to do a full restore, I chose to just reset the location warning data using the setting application, followed by a reboot.
This works for a while although not perfectly.
Lately, doing the above would not zero effect on my ability to improve the signal. I just can't get a lock at all, and when I did, it was extremely unstable, and my position was all over the place with both TomTom and Sygic.
I then did a full restore, which solves everything once again (this is full restore, jb, and sbsettings reinstalled).
Except it lasted only two trips, and it started becoming cranky again.
I'm about to pull out my hair over this.
Next I'm going to uninstall SBsettings and see what happens.
Update: removal of sbsetting does not improve the bad signal. Next, I'm going to un-jailbroken it and see whar happens.
Performance without jailbreak
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Update: Two days now. 5 trial runs and it's perfect. Very fast position locking never experienced before. Once once did the position jumped to another street, which is accuracy never seen before. Will continue to test and report the result.
Day 5, still running perfectly. There were brief moments of cursor being in the wrong street but very quickly it came back.
GPS performancw when Jailbreak again
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Today (day1), I decide to rejailbreak and observe the performance under this situation. I got problem immediately with the reception but the problem is not serious yet. My cursor position went to the wrong street, my cursor seems to be lagging far behind my actual locations, and I had grey and white map again. Also, in inside my house, I get only 2 bars shown on Sygic instead of 3 when I removed the jailbreak.
Day 2: Seems to be working perfectly today.
Day 3: Had a couple of instance where the signal was lost but was mostly very good
Day 4: Morning run was good. Home trip was a complete disaster. Kept getting poor GPS signal message, grey map, and location was jumping all over the place again. Did a location warning reset and reboot. Helped to get the signal, but few seconds later it was grey map and jumping cursor problem for the rest of the trip. Absolutely unuseable.
Day 15: By today, the GPS signal has become COMPLETELY useless. Zero signal no matter how long I stay under the open sky. Not even getting weak signals. Repeat location resets followed by reboots ( and doing it 5 to 6 times in a row) does not cure the problem. Not a single GPS software (Map, TomTom, Sygic, iGO, Copilot, Ndrive, Navigon, Motion X) are getting any signal. I mean NO signal. Not even weak signal.
Day 16 : Do a full restore, then jailbreak immediately. Observation: No longer was able to get signal from my usual position inside the house, but no problem once I go under open sky. So, it is NOT as good as if I stay away from jailbreak.
Jailbreak DOES affect the signal.