I would suggest checking the following:

1. Insure your battery connections on the model are tight and are not vibrating loose when the engine is running

2. If the servos, receiver and transmitter are very old I would suggest sending them in to Hobby Services to be tested, retuned and/or reconditioned

3. The clutch shoes to clutch liner should not be a tight fit. You should have a clearance/gap of about 0.5mm. To install properly, take the clutch show and wrap it twice with electrical tape so that the top edge of the tape is flush with the top of the clutch shoes, then clean out any old glue on the clutch bell, clean the inside of the clutch bell with acetone or other cleaner that doesn't leave any resideu, mix up some 30-minute epoxy and put a smear coat on the inside of the clutch bell, then carefully insert a new clutch liner making sure the ends cleanly butt up against eachother, then slowly push the clutch shoe assembly in place and let it fully cure. The electrical tape will press the liner in place and set a proper gap.

4. Check and/or change your glow plug. Also insure that your glow extension is not intermittant, if in doubt, replace it.

5. Load check your receiver pack to insure its good

6. Make sure your engine bearings are in good shape. MRC no longer carries parts for Enya, so if you need anything for that motor you'll have to go to Enya directly

7. Make sure your receiver crystal is tightly secured in the receiver. If it is loose, you may lose reception when it is exposed to the engine vibration during normal running.

8. Check to make sure your receiver power switch is not intermittant, old or fuel soaked. If in doubt, replace it.

That's about all I can think of. If all that stuff is good, I think you'd be in pretty good shape.