Help with slow and freezing MBP

Hey, So my MBP is running really slow. I got it mid last year and it was fine. I could browse the net while also using Photoshop, and listening to music, no issues what so ever. Now I've noticed in the last couple of months its being stupid slow and apps are freezing(not responding) having me to force quit them.

If I start browsing the web then try to do anything else really it bogs the system down(no multitasking), the only way to get it back to usable speed is to restart. but I don't want to be doing that every hour or so.

Another this is if I have a few different apps open and don't use one for about 10 mins it stops responding and I have to force quit it. I have tried a few of the "speed you mac up" options and cleared my caches and there's about 100gb of free space on my HD so I'm just not sure what's going on. Are newer updates of every application just more power hungry now? do I need to upgrade my ram?

Any ideas or opinions would be appreciated. Thanks

Jeff

System specs :

osx v 10.7.4

Processor 2.2gx i7

ram 4gb 1333 MHz DDR3

500g HD( 100ish is free)