If the standard one for your laptop is 120w and your using 90w ones (even a good 90w one) your going to cook it, its going to be working too hard. they have 120w for a reason
Bigger is better though a 180w will not have to work as hard, wont get as hot and will probably outlive any 120w at that usage
Not that I'm recommending cheap laptop power bricks, but I went to Ebay and got one for my laptop for less than $10 and it's worked flawlessly for years except for getting a little warm (I matched its amperage and such exactly to my laptop's needs to be sure it was compatible). I suggest matching the power brick to the laptops needs rather than buying an inadequate unit or an excessive unit because that's what has worked for me.