As this machine dates back to the late 1990s, I would suspect that the clutch has worn to the point of needing its standing air gap adjusted.
As the clutch wears, the space (air gap) between the clutch rotor and the clutch stator increases. The increase in air gap makes it harder for an aging clutch coil to "grab" the rotor disc and draw it into engagement.
If the clutch does stay engaged after you finally get it to engage, I will say too much air gap is the likely reason.
If it was due to a poor or loose connection, you would probably have worded your complaint something like "The clutch kicks in and out when I'm trying to mow".
Bill Kapaun posted a link to the Warner Electric clutch manual recently, perhaps he will repost it here.
Or, you can Google Warner Electric P-1097-6 and get the manual that way.

Thanks mownie-
I'd lost the bookmark

Here is a link that might be useful: Clutch PDF

Thanks, I'll check that out this weekend. Someone suggested a bad key switch - is this a possibility? How would I check that...the switch has 7 terminals....marked G,L,S,M,B, A1 and A2....I assume (I know, never assume) that the A1 terminal is for the PTO, correct. It's been a long time since I've read schematics!!

If you had a separate and additional source of power feeding the circuits that are fed by the A1 terminal on the key switch, and something interrupted the power coming from terminal A1, the circuits would not be affected, and everything would keep right on working.
And, as it turns out.......there is a separate and additional source of power feeding the circuits when the engine is running, it's called the alternator. So, if there was something iffy about the key switch, it would not matter as the charging circuits would still keep things furnished with power.
And, there are other components that drink from the same stream as the PTO clutch, namely a fuel solenoid valve and an operator presence relay.
If there was a problem in the circuits affecting the PTO clutch, I would expect the other components to suffer also, and when those suffer, the engine shuts down.
Just to keep you from following a dead end suggestion.

Thanks.........make sense. I'll check out the clutch this weekend.

Actually, a "flaky" Operator Presence Relay could cause the problem.
The PTO ground circuit is switched by the OPR.
IF it had a bad contact to pin 87 (either connector or OPR itself), the PTO wouldn't turn on.

Yes, that is a possibility. Would be easy enough to check the relay and connector socket for signs of corrosion or even a loose wire in one of the terminal bays on the connector socket body.
Even the relay itself is cheap enough (if you buy it at an auto parts house) to try a new one.
Nothing special about the relay, no need to go to Sears.
It's a common Bosch 5 terminal, 30 amp relay used around the globe in most everything.

OK, here's where I am right now.....because I was looking for a "cheap fix" I replaced the PTO switch and the operator presence relay, I set the air gap on the clutch to 0.012" and things were no different. So I dug out my old multi-meter and checked the clutch coil resistance and nothing - the needle didn't move at all on my meter. So it looks like I'll be ordering a new clutch soon. What has me a bit baffled is that before it went completely dead it was "iffy" if it would turn on or not, more times than not it would work.....guess that is possible with a failing clutch??

Oh yes, of course that is possible,........in the beginning stages of failure.
If you are using the Ohmmeter to check for resistance, and you get no reading at all, the meter is showing "infinity", or infinite resistance. The explanation for that in regard to a circuit is "the circuit is open, there is no continuity".
This type of failure often begins with intermittent operation of the device the circuit powers, but because the broken spot in the circuit is a point of high resistance, that spot overheats and burns up some of the conducting material of the wiring.
As it worsens, it will soon begin to "arc over" at the break, and the device may even continue to work. Complete loss of continuity follows quickly when that happens, and the device becomes inoperative.
But, with PTO clutches, air gap adjustment is often the simple reason for a clutch being recalcitrant, and so the air gap dimension should always be the first thing to check when intermittent operation comes up.