OS X V10.6 Snow Leopard :: Intermittent Failure To Boot After Installing SSD?

I have recently installed new RAM and an SSD in my late 2009 MacBook Pro, running OS X 10.6.8, and it now frequently hangs when I try to start it up. Here are  details: 

In recent weeks I noticed signs that seemed to point to a failing hard drive. I took the machine to a Genius Bar. The genius tried Disk Utility and a quick diagnostic and could not reproduce an error, but he noted many, many I/O errors recorded in a log file.  

He said I probably had a failing disk, but it might be a cable issue. He gave me two options: leave the machine with Apple overnight to run diagnostics and perhaps repair if the problem was confirmed, or just take a chance and replace the drive myself, for less money and less downtime.  

I went to a local computer store, and the salesman there noted that I was using relatively little disk space. He suggested that I swap my failing 160 G disk for a smaller, 120G solid-state drive, which happened to be on sale. I agreed and also bought some new RAM.  

I installed the drive and RAM, partitioned the disk (with HFS journaling), and restored my files with Time Machine. For that day and the next, everything worked perfectly.  

The second day after the installation, the system hung while starting up. The Apple logo appeared, the gray spinner appeared, and it just spun. I tried resetting PRAM. I tried safe restart. I started up from the install disk and ran Disk Utility: no problems found. I repaired permissions. I took out the new RAM and put back the old. In some cases after trying these steps, the system booted fine — once. In others, it would not boot the first time, but would boot on the second or third attempt. I reinstalled OS X and updated software. The problem persisted. I installed some freeware to enable trimming the new disk. No better.  

I have booted many times in verbose mode. During the unsuccessful boots, there is usually a message "Bug: launchctl.c: 3557 (23930):6 ioctl(S6, SIO CAIFADOR_IN6, &ifra6) ! = -1". The next line is the command "fsck_hfs", and the machine hangs. Eventually it sometimes spits out "launchctl: please convert the following to launchd: /etc/mach_init.d/dashboardadvisory.plist". If I let it go for a long time, I sometimes come back to find a long string of I/O error messages, to the effect that media was expect to be there but wasn’t. I think every time I have gotten the launchctl message, it has booted successfully on the next try.  

During the successful boots in verbose mode, everything happens so quickly that it is hard to say what is going on, but I don’t think it is running fsck_hfs. If it boots successfully and I give it a heavy I/O task, like doing a virus scan, everything works fine. If I run Disk Utility off the installation disk, it says the disk appears to be OK.  

I started the machine in single user mode and ran fsck manually. It gave me reports similar to running Disk Utility, and told me the disk appeared to be OK. It then gave me the prompt for a new command. I typed "exit". It responded "logout". And hung. 

One last thing: I just tried a couple of times to restart with the Apple Hardware test. Despite holding down the D key, the test failed to load, and after a long pause, the machine attempted a normal start-up.  

Info:
MacBook Pro, Mac OS X (10.6.8)