I agree with grump, you can buy a compression tester for about 30 bucks and do it yourself. If you can change a sparkplug you can do a compression test. Stores like autozone often even let you rent tools like that for no charge (just leave a deposit).
Pull all the plugs, pull the fuel pump relay out, put the gauge on one cylinder at a time and crank the engine for about 5 seconds. Write down each number. Then do it again but before attaching the gauge pour about a tablespoon full of oil in each cylinder first. Write down all the numbers again.
You will now have relative dry, and wet numbers to compare.
If the dry test shows for example 5 cylinders with 125 psi or close and one cylinder at 85, you know its an isolated issue. Do the wet test and the numbers should all go up slightly. If the low cylinder does not go up, its a head/valve issue. If the low cylinder comes up drastically, you have worn rings or a damaged cylinder.
If everything is low across the board on both tests the engine is just worn out.
My money is on a single cylinder issue with the head (leaking valve or blown gasket).