My relay wont rev past 2000rpm after 10 mins of use?

My van wont rev past 2000rpm after 10 mins of use we think its the flow sensor. I have had it checked and about 10 faults came up but I think its because the speedo dosen't work ( it blows a 15a fuse ) and that caused the a fault warning light on the dash. I am not to worried about the speedo but I need to have the van running. Any help would be great.