Hiccups and crashing! MineCraft!?

Alright, so I'm one of those many individuals experiencing difficulties while trying to play minecraft. I've been playing around with mods quite a bit lately (shh) and have been experiencing osme hiccups as well as crashing. Is this soley the mod, or is it my computer as well? The game starts up fine, in the first minutes of playing, it is fast. The average fps for the first few minutes is between 90-130. After that, I'm lucky if I can sustain 20 for longer than 3 seconds. It seems like a pretty common issue, as my research has shown. Though, it can be stated that in my case, these problems have not always persisted. Using the same mods, I experienced quite a drastic decrease in lag only earlier today. After reinstalling said mods however, my performance dropped. The first time, with 52 mods, I averaged about 60 fps and experienced no hiccups. Now, with 8 mods, I average 15 fps and hiccup every 3-5 seconds. Any advice or tips on how I can get rid of this? I will try installing the mods again, as well as getting those two performance mods.. the fog and general performance. Past that, I'm not sure what to do. Anything would help. Thanks.