I think you've hit the nail on the head. About halfway through the third season I found myself getting bored - BORED! And the first two seasons were so funny!
The guys getting stoned out in the desert- there was so much potential there! But it was dreadful. Only Raj eating the world's lettuce got a Peter Costello smirk out of me. Howard having sex with his cousin didnt register as funny at all, in fact I got irritated - physically irritated - by the fact that the writers were obviously searching for new lows for this character to reach.
Leonard's mum getting drunk - not funny. Will Wheaton - someone shoot this man please. The comic book store guy reduced from the confident artist to a whimpering shadow of his former self - who's brilliant idea was that? Death to the third season! DEATH I SAY