Oh god, you could be talking about my 5 month old (see other thread on here about exhausting 5 month old)!
He's got worse and worse with daytime napping, and will often howl and thumb suck for 15 minutes or so, despite me comforting him (holding or stroking him whilst he lies in the cot), before falling asleep, only to wake up 20 minutes later.
Today he had a 20 minute nap this morning, interspersed with whining and thumb sucking, and then 20 minutes at lunch, before passing out on me half way through his mid-afternoon feed, only to wake again 10 minutes later. At bedtime tonight he was clearly knackered and probably over-tired, and screamed blue murder for 10 minutes before passing out in his cot.
He will have these little naps and then seemingly wake right-as-rain, smiley, and raring to go again, before flagging and getting grumpy a short while later. Still doesn't sleep through the night either.
No advice i'm afraid but I SO sympathise! Looking for a solution too! Unlike you though the pre-nap howling has started to just cut through me and I can handle it as I know that he is tired and needs the sleep, it's very frustrating though and wearing, and I wonder why he bloody fights it the whole time!