How old is the watch?
When the lubricants in a watch start to evaporate and dry out friction starts to have a bigger influence on the movement of the gear train and escapement. The amplitude of the watch begins to fall. This means, that little wheel that you see in a watch that rocks back and forth doesn't swing around as far as it is supposed to.
A well serviced movement will have a swing (amplitude) of 270-310 degrees when lying flat. As the amplitude decreases it locks and unlocks the escapement quicker because it has less distance to travel.
This causes the watch to start gaining time.
Coupled with the increase in friction in the mainspring due to lack of lubricant, the mainspring doesn't output as much power.
The symptoms you describe indicate a service is needed.
A proper service on that watch would be in the region of 80-90 pounds, that should include disassembly including removing the shock jewels, cleaning in a proper cleaning machine, reassembly using correct spec lubricants, replacing the crown and back gaskets, regulating the watch and pressure testing it.
This is one of the pitfalls of mechanical movements, a service like this on a R olex at a approved agent would probably be around £400
I would not recommend trying to regulate the watch yourself, the regulator will be able to adjust discrepancies (of a serviced watch) in the region of a few seconds, when the watch is minutes out it is time for a service.
My tutor (I am training in Horology) has his own business http://www.clock-watches.co.uk/who-are-Antique-&-Modern
I am in class tomorrow and I will ask him what he charges on a Seiko service. He is very good