my buddy has a similar problem with his 25 merc,the only thing fishy was the primer bulb was going soft when the motor started to slow,he took off the cover and at wot you could see the gas drain out of the fuel filter till there was almost none.we are pretty sure it was the 0 rings on the gas line at the tank or motor or both were sucking some air ,the line was original and very hard so he replaced but he hasn't been out to test it yet.
Basically just a SIMPLE PLUMBING JOB
vance