Have you seen this link on A times?
http://babywhispererforums.com/index.php?topic=64158.msg476652#msg476652Generally 45 minute naps are undertired (UT) and 30 minute naps are overtired (OT). It's a vicious cycle, as you say because one short nap can lead to an OT day! I found that getting my DD on the right A times really helped. About the tired signals, it is important to watch your baby and not just the clock, but they can be confusing. Are you sure he is tired when he fusses? My DD fusses all the time, often just because she is bored, or wants to be held, or whatever.
BTW, it does get better! I started my DD on EASY around the time she turned 5 months. Once I got her A times right and had the routine going, I still had to help her extend her naps a lot of the time but she often did go back to sleep with help! And I think she sometimes slept straight through on her own! Also, around 6 months babies often naturally start sleeping longer on their own... we dealt with the short nap monster for FOREVER but now (at 8 months) she takes two nice long naps almost everyday!
You're at the hard stage, but hang in there and life will get so much better!