Half of the time my iMac Wont boot?

[iiamthelaw]

Im new here so im sorry if this is in the wrong section( My First Post!) I have been having an issue ever since I upgraded to Lion. The problem is that 50% of the time my Mac Wont start. What happens is I start the computer up and hear the chime and then the gear pops up and doesn't stop spinning. Selecting the boot drive by holding alt seems to increase the odds but its getting annoying and quite scary now. A few weeks ago I seemingly found a solution by selecting the boot drive in system setting but the problem has reoccured. Strangely, when the computer is on it works fine but it can be a bit sluggish at times, but I assume this is because i use quite memory intense programs and only have 2 gb of ram ( i bought 8 the other day and should arrive any day now) any help would be appreciated

Specs: iMac Early 2009
2.66ghz processor
2 GB 1067 MHz DDR3
NVIDIA GeForce 9400 256 MB

Thanks again for any help

 

[simsaladimbamba]

What Mac OS X version do you use?

Have you tried any of these steps?

Mac doesn't boot

Resolve startup issues and perform disk maintenance with Disk Utility and fsck

Troubleshooting: My computer won't turn on

A flashing question mark appears when you start your Mac​

 

[iiamthelaw]

I have tried a few of those in the first link, Specifically
PRAM Reset
Safe Boot
and there might be a few others but i cant remember
Im Running Mac OSX Lion 10.7 no updates or anything.

Also I do have a few crashes every now and again, I forgot to mention in the initial post. Thanks for the reply, Ill have a read through the other links.

Also, I have tried fixing permissions from both the Recovery HDD Disk Utility and the on on the OS last time this happened but it didn't fix anything.

 

[iiamthelaw]

I tried resetting the SMC as suggested in one of the links and it seems to have solved the problem(as well as a noticed speed increase) I'll be keeping an eye on it for a while but hopefully its sorted. Thanks for your help, Much appreciated